There used to be this international Coca-Cola display at Epcot (not sure if it's still there) here in Orlando, and had some flavors of Coke products from other countries, stuff that wasn't marketed here in Orlando. Some of that stuff was god awful, can't recall any names.
Oh, and btw, if it hasn't been mentioned yet. Minute-Maid's diet lemon-aid, is OMG horrible as well.
Just remembered another, though you have to combine a very common snack (Cheetos) and another common, normally good tasting beverage (Hawaiian Punch) for the desired effect. Eat a bunch of Cheetos, not bad eh? Now try to wash it down with some Hawaiian Punch. For some reason, this combo creates a god awful taste in your mouth. I discovered this in High School during lunch breaks. Eeeek, what a nasty taste.