1. Eating an extremely hot curry
2. Having un-lubed anal sex
3. Someone has put a flammable substance on your butt and your ass begins burning and blisters your butthole
"Your such an idiot, why didn't you use lube?! Now your buttfire has melted the condom to my cock and your butthole."
"Haha, yeah we made a buttfire on Paul, it was cold last night."