I think I was using Grok Code 1 Fast with Cline, and had it trying to fix some code. Came back a bit later and found out that after not being able to make progress on fixing the code, it decided to "fix" the test by replacing it with a trivial test.

That made the test pass of course, leaving the code as broken as it ever was. Guess that one was on me though, I never specified it shouldn't do that...