diff --git a/Cargo.lock b/Cargo.lock
index b95f982..a22bc1f 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -150,7 +150,7 @@ dependencies = [
  "heck",
  "proc-macro2",
  "quote",
- "syn 2.0.11",
+ "syn 2.0.13",
 ]
 
 [[package]]
@@ -171,6 +171,7 @@ dependencies = [
  "itertools",
  "jwalk",
  "path-clean",
+ "predicates",
  "rayon",
  "regex",
  "rusty-hook",
@@ -321,6 +322,15 @@ dependencies = [
  "rustc_version",
 ]
 
+[[package]]
+name = "float-cmp"
+version = "0.9.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "98de4bbd547a563b716d8dfa9aad1cb19bfab00f4fa09a6a4ed21dbcf44ce9c4"
+dependencies = [
+ "num-traits",
+]
+
 [[package]]
 name = "fsio"
 version = "0.1.3"
@@ -441,9 +451,9 @@ checksum = "99227334921fae1a979cf0bfdfcc6b3e5ce376ef57e16fb6fb3ea2ed6095f80c"
 
 [[package]]
 name = "linux-raw-sys"
-version = "0.3.0"
+version = "0.3.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "cd550e73688e6d578f0ac2119e32b797a327631a42f9433e59d02e139c8df60d"
+checksum = "d59d8c75012853d2e872fb56bc8a2e53718e2cafe1a4c823143141c6d90c322f"
 
 [[package]]
 name = "log"
@@ -484,6 +494,12 @@ version = "0.5.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ab250442c86f1850815b5d268639dff018c0627022bc1940eb2d642ca1ce12f0"
 
+[[package]]
+name = "normalize-line-endings"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "61807f77802ff30975e01f4f071c8ba10c022052f98b3294119f3e615d13e5be"
+
 [[package]]
 name = "nu-ansi-term"
 version = "0.46.0"
@@ -494,6 +510,15 @@ dependencies = [
  "winapi",
 ]
 
+[[package]]
+name = "num-traits"
+version = "0.2.15"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "578ede34cf02f8924ab9447f50c28075b4d3e5b269972345e7e0372b38c6cdcd"
+dependencies = [
+ "autocfg",
+]
+
 [[package]]
 name = "num_cpus"
 version = "1.15.0"
@@ -536,8 +561,11 @@ checksum = "c575290b64d24745b6c57a12a31465f0a66f3a4799686a6921526a33b0797965"
 dependencies = [
  "anstyle",
  "difflib",
+ "float-cmp",
  "itertools",
+ "normalize-line-endings",
  "predicates-core",
+ "regex",
 ]
 
 [[package]]
@@ -558,9 +586,9 @@ dependencies = [
 
 [[package]]
 name = "proc-macro2"
-version = "1.0.54"
+version = "1.0.55"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e472a104799c74b514a57226160104aa483546de37e839ec50e3c2e41dd87534"
+checksum = "1d0dd4be24fcdcfeaa12a432d588dc59bbad6cad3510c67e74a2b6b2fc950564"
 dependencies = [
  "unicode-ident",
 ]
@@ -633,9 +661,9 @@ dependencies = [
 
 [[package]]
 name = "rustix"
-version = "0.37.5"
+version = "0.37.6"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "0e78cc525325c06b4a7ff02db283472f3c042b7ff0c391f96c6d5ac6f4f91b75"
+checksum = "d097081ed288dfe45699b72f5b5d648e5f15d64d900c7080273baa20c16a6849"
 dependencies = [
  "bitflags",
  "errno",
@@ -692,7 +720,7 @@ checksum = "4c614d17805b093df4b147b51339e7e44bf05ef59fba1e45d83500bcfb4d8585"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.11",
+ "syn 2.0.13",
 ]
 
 [[package]]
@@ -742,9 +770,9 @@ dependencies = [
 
 [[package]]
 name = "syn"
-version = "2.0.11"
+version = "2.0.13"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "21e3787bb71465627110e7d87ed4faaa36c1f61042ee67badb9e2ef173accc40"
+checksum = "4c9da457c5285ac1f936ebd076af6dac17a61cfe7826f2076b4d015cf47bc8ec"
 dependencies = [
  "proc-macro2",
  "quote",
diff --git a/Cargo.toml b/Cargo.toml
index 980e6f2..20056cd 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -24,3 +24,4 @@ tracing-subscriber = { version = "0.3.16", features = ["env-filter"] }
 [dev-dependencies]
 assert_cmd = "2.0.10"
 rusty-hook = "^0.11.2"
+predicates = "3.0.2"
diff --git a/src/ext.rs b/src/error_stack_ext.rs
similarity index 100%
rename from src/ext.rs
rename to src/error_stack_ext.rs
diff --git a/src/main.rs b/src/main.rs
index 56e18d4..4e7e971 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,4 +1,4 @@
-use ext::IntoContext;
+use error_stack_ext::IntoContext;
 use ownership::Ownership;
 
 use crate::project::Project;
@@ -13,7 +13,7 @@ use std::{
 };
 
 mod config;
-mod ext;
+mod error_stack_ext;
 mod ownership;
 mod project;
 
@@ -85,7 +85,7 @@ impl Context for Error {}
 
 fn main() -> Result<(), Error> {
     install_logger();
-    print_validation_errors_to_stdout(cli())?;
+    maybe_print_errors(cli())?;
 
     Ok(())
 }
@@ -99,14 +99,13 @@ fn cli() -> Result<(), Error> {
 
     let config_file = File::open(&config_path)
         .into_context(Error::Io)
-        .attach_printable(format!("{}", config_path.to_string_lossy()))?;
+        .attach_printable(format!("Can't open config file: {}", config_path.to_string_lossy()))?;
 
     let config = serde_yaml::from_reader(config_file).into_context(Error::Io)?;
 
     let ownership = Ownership::build(Project::build(&project_root, &codeowners_file_path, &config).change_context(Error::Io)?);
-    let command = args.command;
 
-    match command {
+    match args.command {
         Command::Validate => ownership.validate().into_context(Error::ValidationFailed)?,
         Command::Generate => {
             std::fs::write(codeowners_file_path, ownership.generate_file()).into_context(Error::Io)?;
@@ -120,7 +119,7 @@ fn cli() -> Result<(), Error> {
     Ok(())
 }
 
-fn print_validation_errors_to_stdout(result: Result<(), Error>) -> Result<(), Error> {
+fn maybe_print_errors(result: Result<(), Error>) -> Result<(), Error> {
     if let Err(error) = result {
         if let Some(validation_errors) = error.downcast_ref::<ownership::ValidatorErrors>() {
             println!("{}", validation_errors);
diff --git a/src/ownership/validator.rs b/src/ownership/validator.rs
index b35be6b..9e916c7 100644
--- a/src/ownership/validator.rs
+++ b/src/ownership/validator.rs
@@ -157,6 +157,8 @@ impl Error {
 impl Display for Errors {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         let grouped_errors = self.0.iter().into_group_map_by(|error| error.error_category_message());
+        let grouped_errors = Vec::from_iter(grouped_errors.iter());
+
         for (error_category_message, errors) in grouped_errors {
             write!(f, "\n{}", error_category_message)?;
 
diff --git a/src/project.rs b/src/project.rs
index ab4c22f..a70d324 100644
--- a/src/project.rs
+++ b/src/project.rs
@@ -13,7 +13,7 @@ use rayon::prelude::{IntoParallelIterator, ParallelIterator};
 use regex::Regex;
 use tracing::{debug, instrument};
 
-use crate::{config::Config, ext::IntoContext};
+use crate::{config::Config, error_stack_ext::IntoContext};
 use glob_match::glob_match;
 
 pub struct Project {
@@ -131,7 +131,7 @@ impl Context for Error {}
 impl Project {
     #[instrument(level = "debug", skip_all)]
     pub fn build(base_path: &Path, codeowners_file_path: &Path, config: &Config) -> Result<Self, Error> {
-        debug!("scanning project ({})", base_path.to_string_lossy());
+        debug!(base_path = base_path.to_str(), "scanning project");
 
         let mut owned_file_paths: Vec<PathBuf> = Vec::new();
         let mut packages: Vec<Package> = Vec::new();
diff --git a/tests/fixtures/invalid_project/.github/CODEOWNERS b/tests/fixtures/invalid_project/.github/CODEOWNERS
new file mode 100644
index 0000000..8b13789
--- /dev/null
+++ b/tests/fixtures/invalid_project/.github/CODEOWNERS
@@ -0,0 +1 @@
+
diff --git a/tests/fixtures/invalid_project/config/code_ownership.yml b/tests/fixtures/invalid_project/config/code_ownership.yml
new file mode 100644
index 0000000..fa56397
--- /dev/null
+++ b/tests/fixtures/invalid_project/config/code_ownership.yml
@@ -0,0 +1,10 @@
+owned_globs:
+  - "**/*.{rb,tsx}"
+ruby_package_paths:
+  - ruby/packages/**/*
+javascript_package_paths:
+  - javascript/packages/**
+team_file_glob:
+  - config/teams/**/*.yml
+vendored_gems_path: gems
+unowned_globs:
diff --git a/tests/fixtures/invalid_project/config/teams/payments.yml b/tests/fixtures/invalid_project/config/teams/payments.yml
new file mode 100644
index 0000000..af0c841
--- /dev/null
+++ b/tests/fixtures/invalid_project/config/teams/payments.yml
@@ -0,0 +1,5 @@
+name: Payments
+github:
+  team: '@PaymentTeam'
+owned_globs:
+  - ruby/app/payments/**/*
diff --git a/tests/fixtures/invalid_project/config/teams/payroll.yml b/tests/fixtures/invalid_project/config/teams/payroll.yml
new file mode 100644
index 0000000..8c66f53
--- /dev/null
+++ b/tests/fixtures/invalid_project/config/teams/payroll.yml
@@ -0,0 +1,9 @@
+name: Payroll
+github:
+  team: '@PayrollTeam'
+ruby:
+  owned_gems:
+    - payroll_calculator
+javascript:
+  owned_packages:
+    - 'PayrollFlow'
diff --git a/tests/fixtures/invalid_project/gems/payroll_calculator/calculator.rb b/tests/fixtures/invalid_project/gems/payroll_calculator/calculator.rb
new file mode 100644
index 0000000..3505ddb
--- /dev/null
+++ b/tests/fixtures/invalid_project/gems/payroll_calculator/calculator.rb
@@ -0,0 +1,6 @@
+# @team Payments
+class PayrollCalculator
+  def calculate
+    10_000
+  end
+end
diff --git a/tests/fixtures/invalid_project/ruby/app/models/bank_account.rb b/tests/fixtures/invalid_project/ruby/app/models/bank_account.rb
new file mode 100644
index 0000000..e57ee65
--- /dev/null
+++ b/tests/fixtures/invalid_project/ruby/app/models/bank_account.rb
@@ -0,0 +1,3 @@
+# @team Payments
+
+class BankAccount; end
diff --git a/tests/fixtures/invalid_project/ruby/app/models/payroll.rb b/tests/fixtures/invalid_project/ruby/app/models/payroll.rb
new file mode 100644
index 0000000..ac9c4ed
--- /dev/null
+++ b/tests/fixtures/invalid_project/ruby/app/models/payroll.rb
@@ -0,0 +1,3 @@
+# @team Payroll
+
+class Payroll; end
diff --git a/tests/fixtures/invalid_project/ruby/app/payments/nacha.rb b/tests/fixtures/invalid_project/ruby/app/payments/nacha.rb
new file mode 100644
index 0000000..3503dfc
--- /dev/null
+++ b/tests/fixtures/invalid_project/ruby/app/payments/nacha.rb
@@ -0,0 +1 @@
+class Nacha; end
diff --git a/tests/fixtures/invalid_project/ruby/app/unowned.rb b/tests/fixtures/invalid_project/ruby/app/unowned.rb
new file mode 100644
index 0000000..e69de29
diff --git a/tests/fixtures/invalid_project/ruby/packages/payroll_flow/package.yml b/tests/fixtures/invalid_project/ruby/packages/payroll_flow/package.yml
new file mode 100644
index 0000000..270b730
--- /dev/null
+++ b/tests/fixtures/invalid_project/ruby/packages/payroll_flow/package.yml
@@ -0,0 +1,2 @@
+metadata:
+  owner: Payroll
diff --git a/tests/fixtures/valid_project/.github/CODEOWNERS b/tests/fixtures/valid_project/.github/CODEOWNERS
index a60f0a8..c91ffdc 100644
--- a/tests/fixtures/valid_project/.github/CODEOWNERS
+++ b/tests/fixtures/valid_project/.github/CODEOWNERS
@@ -13,7 +13,7 @@
 /ruby/app/models/payroll.rb @PayrollTeam
 
 # Team-specific owned globs
-/ruby/app/payments/**/* @PayrollTeam
+/ruby/app/payments/**/* @PaymentsTeam
 
 # Owner metadata key in package.yml
 /ruby/packages/payroll_flow/**/** @PayrollTeam
@@ -22,8 +22,8 @@
 /javascript/packages/PayrollFlow/**/** @PayrollTeam
 
 # Team YML ownership
-/config/teams/payments.yml @PayrollTeam
-/config/teams/payroll.yml @PaymentsTeam
+/config/teams/payments.yml @PaymentsTeam
+/config/teams/payroll.yml @PayrollTeam
 
 # Team owned gems
-/gems/payroll_calculator @PaymentsTeam
+/gems/payroll_calculator @PayrollTeam
diff --git a/tests/fixtures/valid_project/config/teams/payments.yml b/tests/fixtures/valid_project/config/teams/payments.yml
index 5e18b66..2f01f29 100644
--- a/tests/fixtures/valid_project/config/teams/payments.yml
+++ b/tests/fixtures/valid_project/config/teams/payments.yml
@@ -1,5 +1,5 @@
-name: Payroll
+name: Payments
 github:
-  team: '@PayrollTeam'
+  team: '@PaymentsTeam'
 owned_globs:
   - ruby/app/payments/**/*
diff --git a/tests/fixtures/valid_project/config/teams/payroll.yml b/tests/fixtures/valid_project/config/teams/payroll.yml
index 72b8466..8c66f53 100644
--- a/tests/fixtures/valid_project/config/teams/payroll.yml
+++ b/tests/fixtures/valid_project/config/teams/payroll.yml
@@ -1,6 +1,6 @@
-name: Payments
+name: Payroll
 github:
-  team: '@PaymentsTeam'
+  team: '@PayrollTeam'
 ruby:
   owned_gems:
     - payroll_calculator
diff --git a/tests/invalid_project_test.rs b/tests/invalid_project_test.rs
new file mode 100644
index 0000000..ae14622
--- /dev/null
+++ b/tests/invalid_project_test.rs
@@ -0,0 +1,18 @@
+use assert_cmd::prelude::*;
+use predicates::prelude::*;
+use std::{error::Error, process::Command};
+
+#[test]
+fn test_validate() -> Result<(), Box<dyn Error>> {
+    Command::cargo_bin("codeowners")?
+        .arg("--project-root")
+        .arg("tests/fixtures/invalid_project")
+        .arg("validate")
+        .assert()
+        .failure()
+        .stdout(predicate::str::contains("CODEOWNERS out of date. Run `codeownership generate` to update the CODEOWNERS file"))
+        .stdout(predicate::str::contains("Some files are missing ownership:\n- ruby/app/unowned.rb"))
+        .stdout(predicate::str::contains("Code ownership should only be defined for each file in one way. The following files have declared ownership in multiple ways.\n- gems/payroll_calculator/calculator.rb (owner: Payments, source: team_file_mapper)\n- gems/payroll_calculator/calculator.rb (owner: Payroll, source: team_gem_mapper)"));
+
+    Ok(())
+}