Even though it's not a restaurant, my worst exp had to be a dive bar in Dallas during Texas-OU weekend: the "Across the Street" Bar.

The toilet and sink were both stopped up and overflowed, numerous cups were strewn about half filled with "lager beer" and my personal favorite: entire jumbo shrimp could be seen floating in the toilet.

... I think the 25c pitchers of beer had something to do with it.

At that point, I reasoned, noone would have noticed that I just opened the door and "played fireman."
-g