I recall an interview with Jason Alexander where he was discussing the Parking garage. He said that at the end, they were supposed to get in the car and drive away. The engine not starting was not planned, and Jason realized that that is the perfect ending for the show. He said he was laughing uncontrollably, and you can see him covering his face and shaking when I watched it on tape.
And when George's mom was in the hospital after "The Contest", she was saying she was hungry, and George threw the Tic-Tacs on the bed, that too, was unplanned.
Anyone know of other unplanned bloopers that made it on the show?