One of my mates from tech had a Beetle. After he bought a decent car, it went to his brother on long term loan. Eventually, his brother decided to get a real car, and the old dak dak was put up for sale - my mate took the opportunity to reclaim the good stereo he'd fitted years before.
Halfway through the process of removing the head unit, he smelled fuel.
Turns out that he'd put a self-tapper through the wall of the fuel tank when he'd fitted it, and removing it unplugged the hole and it was now pouring out of the tank and into the back of the dashboard...
He said "that explained why I started smelling petrol after I installed the stereo".