The following game is certainly a contender for "worst chess blunder ever", especially when you consider the circumstances...
GM Boris Ivkov was on the brink of the greatest success of his tournament career. The former World Junior Champion had succeeded brilliantly in his last tournament, taking clear first place at Zagreb ahead of GMs Larsen, Petrosian, Portisch and Bronstein. Now he was apparently running away with the Capablanca Memorial tournament at Havana, leading the field by a full point after defeating both Bobby Fischer and Smyslov in their individual games.
Two rounds from the end, he faced one of the local rabbits, Garcia...
Ivkov was never quite the same after this game. In his next major tournament, Santa Monica '66 he placed next to last, and was never really a world-class contender again.
How to get the worst chess game:
Teach 2 kids (preferably 6-9) how to play chess, and have them play each other.
It is complete agony and will likely last a very long time.