It used nonsensical parameters to the API in way that I didn't realize was possible (though obvious in hindsight). The AI got confused; it didn't think the parameters were nonsensical. It also didn't quite use them in the way that triggered the error. However it was close enough for me to realize that "hey, I never though of that possibility". I needed to fix the function to return a proper error response for the nonsense.

It also taught me to be more careful about checkpointing my work in git before letting an agent go wild on my codebase. It left a mess trying to fix its problems.