Corrected entry: Throughout the whole show, George Costanza is showed as being left-handed (eg. at the beginning of the episode titled "The Shoes", Jerry and George are writing "The Butler" script, and George can clearly be seen writing with his left hand). However, the actor himself is not, and you can spot some episodes (mostly in the first seasons) but a few in the last seasons as well, with him using his right hand for writing or eating, while in most of the show he's using his left-hand for everything.
Corrected entry: I've always wondered how Kramer is able to live in a New York City/Manhattan apartment. The guy has no job, no prospects of getting a job, doesn't seem to have money in the bank (or at least enough of it to cover the costs of living in his apt) and yet can afford to live there. To the best of my knowledge it is never explained how he's able to afford it. [This is supposed to be one of the eternal mysteries of Kramer. In fact, Jerry and Elaine discuss how great it must be to be Kramer in one episode ("He meets all these beautiful women, he falls a**backwards into money...").]