I had the AI implement two parallel implementations of the same thing in one project. Was lots of fun when it was 'fixing' the one that wasn't being used. So yeah, it can definitely muck up your codebase.
It worked fine until recently where I will ask it to tweak some behavior of a command with a flag and it does a diff with like hundreds of lines. So now it's struggling to catch every place it needs to change some hardcoded duplicate values it decided to copy/paste into two dozen random places in the code.
To be fair it is doing a decent job unfucking it now that I noticed and started explicitly showing it how ridiculously cumbersome and unmaintainable it made things with specific examples and refactoring. But if I hadn't bothered to finally sit down and read through it thoroughly it would have just become more broken and inconsistent as it grew exponentially.