I saw the finale the other day, and I noticed something strange. In the sequence where the Seinfeld theme plays in the background, and we see all the minor characters from past episodes pacing around, waiting for the jury's verdict, there is one shot of Elaine's rabbi friend giving a sermon (I'm not sure if "sermon" is the right word in a Judaic context), and the people listening to him are Jerry's two parents, and Frank Costanza. This is somewhat odd since Frank, as far as we know, is not Jewish. I realized this was probably a little inside joke, because Jerry Stiller (who plays Frank) is Jewish in real life.
Of course, the other possibility is that Frank, after the failure of Christmas and Festivus, decided that Hanukkah was the only way to go, and converted. Either way, it's surprising to see him sitting next to the Seinfelds without arguing, considering how much they dislike each other.
By the way, I thought the funniest part of that sequence was the moment where Poppie complains about the soup, and the Soup Nazi takes it back. :)
Emile