The water mark on the tall guys pants keeps changing.
The cup that Gunther gives Janine in Central Perk is clearly empty by the hollow sound it makes when hitting the table.
In the first few seconds of the show after the theme song, right after he mentions the origin of the Dreidle, Ross can be seen mouthing words though no sound comes out.
When the director yells cut and Joey yells "Nooo" the camera cuts away from him and shows the side of him and you can see him mouth saying "Nooo" again but with no sound.
Right after Janine invites Ross and Monica, she gets up to leave Central Perk and you can see a camera man behind her.