After Jerry leaves his garbage on Kramer's door and knocks when he closes his apartment door you can see the whole wall shake.
Jerry (Jerry Seinfeld) said that George's (Jason Alexander) parents are bald, however, in future episodes they are played by Estelle Harris & Jerry Stiller - neither are bald.
This is the second episodes in which George (Jason Alexander) mentions having a brother. He is never seen or mentioned again.
In order to get his vacuum back from Martin (C.E. Grimes), Kramer (Michael Richards) tells him his mother is coming to visit and will be appalled at the rug not being vacuumed. In episode 6.11, The Switch (1995) however, Kramer says he hasn't spoken to his mother in five years.