Drift

A session slowly moving away from what you asked: the scope grows, an early rule stops being followed, the goal changes shape. No single turn looks wrong. You notice when the diff has things in it you never asked for.

Your first message becomes a smaller share of what the model reads, compaction may have removed it, and you are probably not watching closely by then. The defence is short sessions, rules kept on disk and not in chat, and comparing the diff with the original request.