Experienced engineers that know the codebase and system well, and with enough time to consider the problem properly would likely consider this case.

But if we're vibing... This is the kind of bug that should make it back into a review agent/skill's instructions in a more generic format. Essentially if something is done to the message history, check there tests that subsequent turns work as expected.

But yeah, you'd have to piss off a bunch of users in prod first to discover the blind spot.