Parallel sessions overwrote each other's work because they shared a working directory

[crowcreation]

I was running 4-5 AI coding sessions in parallel across different tasks in the same repository. Each session checked the current branch at startup and confirmed it was correct. Everything looked fine.

Mid-session, one task created a new branch. Another session's pre-flight check had already passed, so it didn't notice. When session #3 committed, it committed to the wrong branch. Session #4's file edits were silently overwritten when session #2 merged.

Four incidents in a single afternoon. None of them crashed. None threw errors. Each session completed its work successfully in isolation. The failures were invisible until I checked what had actually landed.

Drift mode: State staleness + context fragmentation

What broke: Each session cached its branch state at startup and never rechecked. The assumption "branch hasn't changed since I checked" was true when checked and false by the time the commit happened. No session had visibility into what the other sessions were doing.

How it was detected: Post-merge inspection. One commit included content from a different feature branch. Working backwards from there, found the other three incidents.

What changed: Added a branch verification check that runs immediately before every commit, not just at session startup. The pre-flight check catches divergence at startup; the pre-commit check catches divergence that accumulates during long sessions. Simple rule, but it required failing four times in one day to see why startup-only verification wasn't enough.

Pattern: Pre-flight checks validate a point-in-time snapshot. For long-running sessions where external state can shift, you need validation at the point of action too, not just at the point of entry.