Any given old man in a Stephen King book. Roland, Father Callahan, the guy from Pet Semetary whose name eludes me right now- these are all people who it would be a bad idea to piss off.
Also? Stinging insects. Of any kind. Well, not really any kind, just bees, wasps, and hornets. I had a very bad experience with a nest of bald-faced hornets* back when I was four. Had to be taken to the hospital. Two shots in the ass. It was a bad day, and it only got worse when I had to sleep on all the stings.
*Note: this is the Northwest Pennsylvania name for them. I'm not sure if they're called anything else in other regions, but just so everyone knows what I'm talking about: those huge hornets with the white heads and black-and-white-striped bodies.