Remove the human element. Yes, someone spent time fixing a bug. If the fix doesn't look like it makes sense on its own, do not merge it. If the author tries to convince you that it's a good fix, it's an immediate no.

A good fix (which is the only acceptable fix in open-source software), is one that speaks for itself.

Well, I dunno. Sometimes the fix speaks for itself but the other party is as dumb as a box of rocks and doesn’t understand. It can be hard to tell the difference.