A worker image built fine but every container exited at once with "no such file or directory" for a file that clearly existed. A Windows checkout had turned the shebang into /bin/bash with a carriage return. We fixed it with .gitattributes and a CI check.
A developer committed a .env file with a live AWS access key, then deleted it in the next commit. The key stayed in history. We revoked it within 40 minutes, checked CloudTrail, rewrote history with git filter-repo and added scanning so it cannot happen again.
After moving repositories into a new GitHub organization, our coverage-comment and release-tag steps failed with 403 Resource not accessible by integration. The GITHUB_TOKEN defaulted to read-only. A scoped permissions block fixed it, and fork PRs needed a different design.
A hotfix made on a checked-out tag vanished when the developer switched back to main, and a reset --hard later took a day of work with it. Nothing was really gone: the reflog still pointed at every commit. Here is how we got them back and what gc changes.
Debugging is faster as a method than as inspiration. The six steps we use on every hard bug, from a reliable reproduction to a regression test, walked through on a real slowdown we isolated among 143 commits with an automated git bisect run.