Root Cause

An engineering blog of real bugs hit while shipping small software products — the symptom, the diagnosis that actually worked, and the fix. Playwright, Firebase, Unity, zsh, Blender, and the traps that cost us days.

• Two agent sessions, one working tree: the checkout that ate my work

  A branch belongs to a working tree, not to a session. When one session ran git checkout, the files changed under another session that was still editing them — and a cloud-synced .git made the evidence disagree with itself.

  2026-09-07

• Google sign-in worked on one Firebase site and broke on the next

  Firebase Hosting multi-site domains are not added to Auth's authorized domains automatically. You can fix it from the command line, but the PATCH replaces the whole list, and a missing header reports the wrong error.

  2026-09-07

• The Blender transform you read back is not the one you just set

  Assembled models flew apart, a key's teeth became a giant cube, and a rocket drifted out of frame. Two traps in a headless Blender batch pipeline, both from reading matrix_world before the dependency graph had caught up.

  2026-09-07

• Chasing a magenta material took our Unity build to 590,000 shader variants

  Adding URP's shaders to Always Included Shaders fixed a magenta material and stopped the build from ever finishing. The project had five materials. Here is why that setting disables Unity's shader stripping, and the material-template fix that restored it.

  2026-09-06

• Opening a Playwright profile in real Chrome broke it permanently

  A status tool opened our saved Playwright profiles with channel chrome. Every scheduled job that used those profiles then hung on launchPersistentContext. Chrome had migrated the profiles to a newer version and refuses to go back.

  2026-09-06

• firebase deploy said "Deploy complete". It shipped three-day-old code.

  A Cloud Functions deploy reported success while uploading a stale lib directory, because the build script ended in "|| echo". Fixing that surfaced a second trap — module-scope admin.firestore() crashing at load, hidden by TypeScript's require emit order.

  2026-09-06

• Ctrl-R did nothing in my terminal app. The cause was EDITOR=vi.

  Reverse history search was silently dead in our Electron/xterm.js terminal but worked in Terminal.app. Four layers of key-delivery tracing found nothing. The real cause was zsh switching to vi mode because of an inherited environment variable.

  2026-09-06