Seriously. Once I was assigned to a project with a coworker that commented code instead of deleting it. I explained to him why this was wrong, he kind of agreed

Seriously. Once I was assigned to a project with a coworker that commented code instead of deleting it. I explained to him why this was wrong, he kind of agreed and deleted some of the commented code as he saw it, but didn't care much either way. Until at some point, we needed to test a part of the program without a feature, so we commented the code that was related to it. When the time came to uncomment that code, we realized we had no quick way to know which code we had just commented, and what was old stuff that should have been deleted. After we finished with that, we took a few hours to remove all the useless commented code.