Continuity mistake: When Ross and Rachel are out in the Porsche it is dark outside, but when it later cuts back to Joey in his apartment stealing Rachel's underwear it is broad daylight. It is also light outside when Joey and Phoebe are talking at the coffee house later on.
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: In the scene where they take Chandler to the hospital, Monica hands the bag of ice containing Chandler's toe to the doctor. The doctor is wearing gloves in one shot then no gloves in the next. This goes back and forth three or four times.