Plot hole: When Elaine is searching for Jean Paul in the streets, one of her verbal flashbacks is of Jean Paul saying, "I trust Elaine, she is my friend." However, Jean Paul made this remark to Jerry, and Elaine was not there to hear it. How could she have a flashback of it?
Plot hole: When George and Jerry are discussing the rye bread scheme, George says Susan is working late that night, so she won't be around when he sneaks the rye into the house. But when George reels the rye in with the fishing pole, Susan is standing there with her parents, wearing her coat. When did she get home? She couldn't have entered through the front door, because Jerry was there tossing the rye up to George. And since it's a New York City brownstone, there's no backdoor that's accessible. There's no logical explanation for her getting into the house to see George reeling in the rye.
Plot hole: In the episode where Kramer decides to start sleeping like a Seal, he's with a girl. He falls asleep on top of her and she seems to think that he's dead. While breathing, your chest expands in such a way that if somebody were laying on top of you, there is no conceivable way that you could think that they are dead.