When I got hired at my current job, they handed me an AI generated app. It did a pretty reasonable job on the frontend, I think (I'm not a React guy), but the backend was a disaster. Part of it involved parsing a file and they had somehow fed the AI a test file with the first 20B truncated. I can tell that the AI tried hard to force the parser to match the file spec and ended up inserting checks for magic byte values that made no sense.

It took me a few days to realize what was happening. Once I got some good files it was just a couple hours to understand the problem. Then three weeks untangling the parser and making it actually match the spec.

And then three months refactoring the whole backend into something usable. It would have taken less time to redo it from scratch. If I'd known then what I know now, I would have scrapped the whole project and started over.

Did you use AI to help you understand the code and what it was doing (incorrectly)?

The AI can generate a lot of Chesterton Fences. It's difficult to figure out why they are there and if they are needed.