Continuity mistake: Ross buys the Ultimate Fighting Combo, a large pop and popcorn at the fight, but when he and Monica sit down, they are never seen again. Furthermore, when Monica talks to Pete at the ring, you can see that the Combo was not set on the floor.
Continuity mistake: At the beginning of the episode, Phoebe gives Monica a push on her rollerskates and then we cut to Gunther giving Rachel a mug of cocoa. At this point, Monica is about to crash into Rachel, but as the angle changes we see Phoebe giving Monica a push for a second time.
Continuity mistake: When the six are looking out the window at Ugly Naked Guy cooking his turkey, they are looking straight across the alley into his window. In "TOW The Giant Poking Device", Ugly Naked Guy's apartment is several floors below theirs in the adjoining building.