When George drives the van to the park with his cousin, it is clearly seen from an overhead shot that a larger man in a brown shirt with an over-sized belly is driving and not George. George is wearing a plaid shirt and his belly is no where protruding as far as shown in the next shot.
The amount of sweat on Newman's face changes between shots when he is arguing with Kramer about the uselessness of mail.
Jerry sees Elaine's boyfriend, Jack, on an old taped commercial and stopped the tape to show Elaine. He didn't rewind it, however, and the footage showed the commercial from the beginning.
The agent that brings Newman to the postmaster was not wearing sunglasses when they entered the room, but in the next shot he is wearing one.
When Kramer and Jerry come in offering to help write the ad to sell the van, Jerry's table has nothing on it as Kramer starts pulling a piece of paper out of his jacket pocket. When we see him starting to write, there is an empty envelope on the table.
At Monk's Kramer (Michael Richards) says that no one was at the post office and Jerry (Jerry Seinfeld) reminds him that it's Sunday. The red plaque next to the cash register has Monk's operating hours and it says "Closed Sunday."
After Kramer shows Jerry the bricks that he placed in his mailbox to prevent mail delivery, Jerry grabs the mail from his mail box and they proceed toward the elevator . As Jerry and Kramer walk towards the elevator , you can see the shadow of the moving boom mic on the upper right corner of the elevator entrance.