I've always heard urban legends of piss water bottles and batteries being thrown and cars being keyed and tires being slashed, but I've never met anyone that's actually said it happened to them.
Most fanbases by and large are pretty similar as they're just made up of comparable, normal people. Some are worse or better than others, but even the worst ones are usually pretty harmless overall, especially if you're respectful and courteous. Not saying that really terrible things haven't happened, just that they're the exception.
Reading through some threads on CU boards really just makes me confused and pity them, though. Like, the joke is on you if your hatred of something so arbitrary and meaningless makes you go hours out of your way to not drive through a state, or refuse to wear or own anything of a certain color, or whatever. It's like a junior high pep rally over there.