My worst experience would have to be one of those inverted Reptar coasters made for little kids. I rode it just so I could add it to my list and then found out ten minutes later I was about a centimetre away from being too tall. It kinda freaked me out that the ride ops didn't even check :/
There's also the Corkscrew here in New Zealand, which is pretty much a minute and a half of headbanging. I never thought it was possible to headbang on a lift hill.