Continuity mistake: In the scene where Ross takes a liquorice from the skull, in the first close up shot he is holding it in his hand, but then immediately after this when they zoom out, he is chewing a piece. This is impossible because he would have had no time to bite it.
Visible crew/equipment: When Monica and Chandler are discussing the date he had with Ginger, when he says, "She's great looking," two stage lights are visible in the background. This mistake is visible for a long period of time, where it's slightly visible, shown in shadows, and largely visible as Chandler and Monica are talking.
Continuity mistake: During the scene when Chandler arrives with his test results, Phoebe is reading a book with a red section on the left hand page. In the next shot, the red on the page has vanished. Then, in the next shot, the red page is back again. Also Phoebe's left hand moves between shots.
Continuity mistake: The way Phoebe holds her flowers when told to smile changes depending on the angle.