Sorry for being a bit vague but I didn't mean to imply there were tests. I meant test in the sense of "do whatever you did to check correctness" in the first place. You change the code, you should do that process again or at least some approximation of it. Automated tests make it pretty easy, manual testing makes it doable, stepping through with a debugger or dumping printing statements makes it painful and error prone.

Many codebases don't have tests at all, most have terrible tests, and under no circumstances do I think they magically prevent all bugs. But I do think obnoxious coders who assume they can just merge and move on without checking the correctness have a special place in hell.