The keeper keeps each transition from its own goroutine, which reads the memory store's Fail field under the store's lock; tests assigned the exported field bare, so TestAnUnreadableStoreClearsNothing failed under -race whenever the goroutine appended in that window. The field is now set only through SetFail (and Told's likewise), and a test makes the race certain rather than rare. Test-only: the controller runs the bus store, never InMemory.