Again not airgun related but was on a now not much used airgun forum.

Guy said he had a friend who replaced the battery on his car with a higher amp hour rating and burned out all the wiring.
He also claimed he could rejuvenate knackered 12volt vehicle batteries and that his system was so good that he had a contract with the army to do this to all the old batteries and they were then returned into service.
He was serious.

Lots of other crazy claims but these two were the worst.