My worst coaster experience was Kong at SFDK.
I went on it with my GP 8th Grade classmates. Ironically, every kid who went on it despised that ride after, and we decided to get our humorous Tivoli credit nearby.
The Top Thrill Dragster launch. Having only seeing front seat POVs on Youtube, I didn't know about the stadium seats. So stupid me craned about the train finding the countdown lights. After eventually putting my head back, the launch was excellent.