The failure mode of returning code that only appears to work correctly is one I've encountered before. I've had Sonnet (4 I think) generate a bunch of functions that check if parameter values are out of valid range and just return without error when they should be a failing assertion. That kind of thing does smell of training data that hasn't been checked for correctness by experienced coders.

Edit: Changed 3.5 to 4.

Edit: Looking back to edits and checkins by AI agents, it strikes me that the checkins should contain the prompt used and model version. More recent Aider versions do add the model.