I remember one game where Fischer gave up a bishop for nothing in return. Don't know his opponent or the date.
The World's Worst Blunder

You cannot define the "worst" mistake in chess because it's a zero sum game. Whichever move leads to an immediate loss can be considered the "worst" moves. I guess you can say there are worst played games if they lead to a loss in fewest moves.

Didn't Fischer win game 2, you know, psychologically?
I like the "you know" very Fischery.
He was a fish. He lost the second game psychologically and practically, then Bush payed Stalin and Spassky lost on purpose.

Take a good look at the link I put in comment #4. This really shows the WORST type possible of blunder - now tell me it isn't so!
Kramnik dropping mate in 1 against Deep Fritz
Bacrot dropping his queen against Inarkiev in the 2008 Grand Prix
At Linares in 2002, Beliavsky made the worst possible move and dropped mate in one in a better position
While it's understandable people want to focus on dramatic invisible checkmates by grandmasters and their ilk, many more games are lost by mistakes than simply good play.
Please help us to find the very worst mistake ever made.