Enzo's barber shop as seen from the outside bears little resemblance to the studio set. In particular the lettering on the window is missing.
Jerry's (Jerry Seinfeld) bike can be seen in the background not moving when Jerry & Gino (David Ciminello) go out to the fire escape. The next shot we see, it's swaying back & forth.
There's a vaccine for polio, but should the disease be contracted, there is no cure for it.
When George (Jason Alexander) is advised that Mr Pensky (Michael Fairman) is there to see him, someone's voice in the audience can be heard exclaiming, "Oh no!"
George says the company he interviewed for is a rest stop supply firm. Later when he's working there, he's given a file of another company to work on. Nothing he does there seems to have anything to do with rest stop supplies.
At the bachelor auction, the first bachelor we see is announced by Elaine as sold to "the lucky lady in the third row". But the previous shot establishes that the audience are sitting at tables, not in rows.