Continuity mistake: Phoebe and Ross are in the soon to be closed bar sitting on the pool table. He clears it of everything, but a few seconds later he stops making out because the "damn balls are in the way" and somehow magics up a handful of snooker balls. If you look closely when he's clearing the table, there are two dark balls at the side which he studiously ignores, and which Lisa Kudrow then surreptitiously grabs and takes onto the table with her so David Schwimmer can grab them later.
Revealing mistake: When Monica offers Joey lemonade and he takes off his clothes and is behind the lamp, the tops of his boxer shorts are visible when he says "Were you just gonna give me lemonade?" Broadcasts may crop off this bottom part, DVD doesn't. You can also see his gray boxer shorts through the plant in the previous shot.