Well I can think of only two that were bad enough to be memorialized in this forum (by far, there are more that I would NEVER try again -even free, but these two really stood out).
My primary care doc (and long time family friend) told me I should try a beer that was popular with him and his army buds when they were in Seattle. I took the wife to Seattle for our (two-years delayed) honeymoon. I love that city, and enjoyed trying many wonderful beers. The beer he remembered with such fondness was called Mt. Ranier. I should have known better when the only place I could find it was in one of the fine local fueling establishments, and it was inexpensive, in a 'tall-boy' can. Bad omens, all. Man I can STILL taste that stuff, and its been 2.5 years! I had the wife take a picture of me holding this poor example of the word "beverage" just to prove I'd bought it.
A number of years prior, back about 17 or so years ago, my friends and I decided to try some beers from 'around the world'. We used to sit around a campfire in the front yard of my fire chief (vol. fire dept) -he had a pretty nice piece of land) and knock back plenty of domestic brew. There was one, I can't recall its' name, but we believed it was from China. It had the graphic of an elephant on the label, and tasted about like what the elephant probably pissed out. Think of carbonated vinegar that probably went bad prior to bottling. We tossed one of the bottles of the stuff into the campfire (bad move) -the resulting explosion knocked the fire out, and glass pieces hit his home -his wife came out and that was the end of our gathering for the week.