The worst thing I ever ate was pork loin that was cooked viking style. The whole piece, with fat, cartilage, tendons and everything, no spices. Rolled into birch bark, put down a hole in the ground, with some hot rocks on top, and then covered with dirt. a few hours later, dug it all up, and got to eat it ... like that... :/ I was about 9 or 10 years old.
It was on a school trip with 8-10 year old kids, and we went to a little viking settlement that is not far from where I live. I mean, sure, show the kids how meat was prepared back then, but have some hot dogs as backup for fucks sake! *mad*
Apart from that, which is a rather special case, I can't remember the worst thing I've eaten. I don't like cilantro / coriander, I'm one of those who think it tastes like soap, but it's not the worst thing for sure.