We used to have a game called toss the quarter at the cap, because we were told by the streetwires rep that a quarter contacting both points on a charged cap would make the cap explode. Anyway, we were horrible shots & finally someone hit it...it did nothing. Then we talked this dumb kid into touching both points with a screwdriver....it did nothing. Then a bunch of us were caravaning down to Indy from MI for a car show, and my buddy had turned his alternator up to 18 volts for the stereo competition before the drive. he was listening to his system really loud on the way down. I just remember seeing him swerve and pull over really fast to the side of the road, open his door and run out of the truck. His 4 farad cap exploded from over voltage. Luckily his box was made of 1" plexi, but it blew a small hole & a huge dent into the back of his cab. It was some pretty mean stuff. That's the only time I've ever seen it, or heard about it where I knew it was real. From what I understand it is pretty rare and is usually from overvoltage, or from a faulty or plugged vent on the cap, which is what I would assume happened in your case budnikguy.