It pains me to know that a lot of people out there who code haven't even heard of a version control system

If you meet people like that, tell them you have something to show them, delete their folders, walk away. They'll figure it out. Lesson learned.

@xsacha too easy, they might have a back-up. Instead, insert subtle bugs here and there, so that they don't find it right now and restore from backup.
By the time they discover the bugs, they won't be able to restore from backups.

It really should be something they teach in school. I didn't even know about it until after college