Some time ago I had a problem that, intuitively, felt similar. An internal inconsistency showing itself as an oddity. PG replied that it happened very shortly

Some time ago I had a problem that, intuitively, felt similar. An internal inconsistency showing itself as an oddity. PG replied that it happened very shortly before the system restarted itself - it had got into a funny state, was doing the best it could, gave up, and restarted. This feels similar, but I have no specific knowledge of the internals, and speak without authority.