I noticed this one of the first times I watched it, but had forgotten about it. It occurs in "The Millenium" episode. Right at the end of the show when Jerry is talking to Newman about Newman's party:
JERRY: By the way Newman, I'm just curious. When you booked the hotel, did you
book it for the millennium New Year?
NEWMAN: (smug) As a matter of fact, I did.
JERRY: Oh, that's interesting, because as everyone knows, since there was no
year zero, the millennium doesn't begin until the year two-thousand and one.
Which would make your party, one year late, and thus, quite lame.
Shouldn't he have said that his party would have been one year early?