When Monica says she needs something old in order to get married, Chandler offers a condom that has been in his wallet since he was 12. In season 4, episode 4 Chandler cleans out his wallet discarding this very condom stating, "the old condom ring in the leather just doesn't scream cool anymore." (Although it's not clear if he's just replacing the wallet or has actually thrown out the condom as well).)
The theory that a slot machine that hasn't paid off in a while is "overdue" is a myth. The payoff odds are programmed to be the same for every pull of the lever regardless of the outcomes of the previous pulls.
Phoebe says she envies Joey for finding his 'identical hand twin', but Phoebe HAS an identical hand twin: her identical twin sister Ursula.
Joey describes the male croupier as 'tall with dark hair', but the croupier is markedly short, several inches shorter than Joey.
When Joey and Phoebe burst into the chapel and Joey is yelling "come on come on!" to Phoebe, it is obvious that Joey's mouth is not moving.
It is said that the marker used by Ross on Rachel's face doesn't come off, but in the following episode (next morning) Rachel's face is clean.