You asked your LLM agent to speed up a folder-move operation in Rails. It writes the textbook idiomatic answer. A whole folder tree goes invisible to its users the next day.

Not a Rails bug. Not a Claude bug. Rails and Claude each did the idiomatic thing, and the idioms collided.

I'll walk three real production incidents from our regulated clinical-trials Rails fleet where this happened – around ActiveRecord callback chains, AR-backed job queues treated as ordinary ActiveRecord tables, and Rails's transaction-by-default. Each trap fires specifically because of the Rails idiom; the convention chain IS the failure surface.

Side-by-side: our actual Rails source, the agent's actual first-pass draft, the skill-layer invariant that catches it before it ships.