Other mistake: There are two mistakes in the scene where Phoebe spots Joey's picture in the laundromat. For one thing, when Phoebe says "Oh, here it is", she is looking in the opposite direction of the picture. Also, when she says this, her mouth doesn't move at all.
Other mistake: When Monica's trying to evoke some kind of an emotional reaction from Rachel, who's polishing her toenails, there's a bottle of Cutex nail polish remover on the table with part of the brand name painted black, so it reads "Cu Ex" on the label.
Add timeSuper Grover
Other mistake: When we see the chess board at Central Perk that Ross and Chandler are using, the black pawn in front of the black knight nearest the camera is missing, and from the arrangement of the white pieces, it cannot have been captured yet.