First I'd trace the trail of clues to the root of the issue using any and all debugging tools available to me. Then, I would test the faulty function in an isolated environment to see what caused the issue and why it wasn't picked up on previously. After finding what the problem is, I'd either fix the broken logic and the chain of logic connected to it(if that was a part of the issue) if I wrote the faulty code myself, or I'd give a heads up about the error to the person who wrote the code along with all the data I gathered about the issue.

Javascript and Python