For me, it's a dead heat between... (drum roll)

1. The coffee fron the coffee truck that came to my job at JLS Computers in the early 80s in downtown Toronto. The guy that drove the truck made unbelievably awful coffee, and I found that out the first time I bought it. It was bloody STRONG...and it tasted sort of like burnt peanuts. I kid you not. Absolutely putrid. I never bought his coffee again after that, but some others kept buying it, to my amazement. ;-)

and...

2. The coffee I bought at Black Cat Fish and Chips, purchased in the mid-70s in Toronto. It was wretchedly weak, stale tasting, and had a distinct flavour of dish washing detergent. I dared the owner, a mustachioed and sweaty Greek with very hairy arms, to drink some of it himself, and he reacted with wounded outrage that I would criticize his coffee! We never reached a meeting of minds on that, I'm afraid. Fortunately, there were several other coffee shops not too far away, and I reminded him of that as I left his establishment.

Now how about the story of your worst cup of coffee?