Continuity mistake: Throughout the entire first scene where Rachel is asking Chandler and Monica if they have seen Joey and he's in the bathroom, Chandler is sitting at the table and his blue cup constantly switches from being in his hand to on the table much too quickly to be real movement. At one point he even holds a piece of food which then disappears from his hand back on to the plate.
Continuity mistake: When Joey is picking up his bagel off the ground in Chandler's apartment he says something about the bagel hole being to small and he has it in his left hand. In the next shot it shows Chandler making a face and Joey's hand is empty. In the next shot Joey is sitting down and the bagel is back in his hand.