I think my all-time worst would have to be a place in Penzance, England - it was on the seafront, attached to an old hotel, the Queen's (once probably lovely, but now with all the ambience of a really down-on-its-luck daycare centre).

I ordered a Margarita (going by the keep it simple, give them less chance to screw up philosophy). It came ice-cold, with a thick coating of sickly yellow tinned sweetcorn, and no cheese.

That was one trip I was very happy to cut short.