Not really a goof, but in the scene where all four of our heroes are featured in Kramer's apartment 'on set', George has just joined them and is telling them how he ran into a flock of pigeons, Kramer says "OK let's take a break!" and proceeds to scoff down some chips and Diet Coke. To watch him do this as the others watch on is absolutely hilarious and I think he burps slightly. At this point, watch George very closely as Jason Alexander can't hold back the laughter anymore! It's quick, but it's there!