Practicing dentistry without a license?
True story...My mom was going to have a root canal and the dentist had put a temporary cap on. And it kept falling off. And each time she went back, he charged her for regluing it. So the third time, she got some non-toxic Elmer's glue and glued it herself.
The dentist went to do the root canal and asked the assistant, "What did I put this on with?". Mom told him what she did, and he replied: "I'm going to have to charge you with practicing dentistry without a license." But he was joking.