Continuity mistake: When Jerry's sitting with George in the hospital talking about George's jacket, as Jerry says "no, I tried it on, it fits good", he's gesturing with his left hand, which he then puts in his lap. From the reverse angle though, his left hand's still raised, then he lowers it again.
Revealing mistake: After the scene in which Elaine tells Jerry about her confrontation with the "head-nodding" man, and before the scene where George swallows a fly, the scene cuts to a shot of the exterior of Monk's café. The actual, real-life restaurant used for exterior purposes is named "Tom's Restaurant," and the "m's" in "Tom's" can be seen during this five-second shot.
Continuity mistake: Jerry says he has never seen an episode of I Love Lucy in his life, then pops the cork on the wine bottle. Then it cuts to the lady and she asks, "Is there anything else about you I should know?" and the wine bottle is still on the table, corked. Then it cuts back to Jerry, and the bottle is still in his hand uncorked.