Prob. the worst thing I ever ate was a bar of chocolate I found in my mother's dresser drawer when I was about 6, which turned out to be Ex-Lax. NOT CHOCOLATE.
My friend's ancient great aunt used to think it really was chocolate, and ask at family reunions "Do you want some chocolate honey??" No one bothered to tell her it wasn't.
There was a trend in Scotland for deep fried Mars Bars, but I think it has mostly passed. The chip shop up the road from me still has an advertisment for them. Needless to say,one try was enough.
I haven't really eaten anything extremely disgusting, but I'd love to hear about other people's experiences.