When Monica comes in and asks Chandler if she can borrow some milk, Chandler is reading a paper that is flat on the bar. In the next shot he is holding it folded, then he lays it on the counter, and suddenly it is in his hands again.
When Monica walks into Joey and Chandler's flat, she points to a bottle on the table and asks if she can borrow it. When the camera angle changes, Monica is holding the bottle with her left hand.
When Ginger leaves after finding Chandler's third nipple, she is taking her coat and her purse is lying on the counter. When the camera angle changes, the purse is suddenly in her hand.
When Joey is telling his story about throwing Ginger's wooden leg in the fire, when he gets to the part about him, Ginger and the dog Pepper falling asleep in front of the fire, alternate camera shots will show Phoebe's hair tucked into her ear and hanging by the right side of her face.
When Joey tries to hide from Ginger behind the coat rack at Central Perk, she futilely calls out his name "Joey?...Joey Tribbiani?!" but she does so with a thick "New Yawk" accident, almost like one of Joey's sisters would have. Yet earlier with Chandler near the bathrooms and later with him at the apartment, Ginger clearly speaks with more of a neutral and/or lack of an accent.
On Mark's last day at Bloomingdales, he agrees to meet up with Rachel on Saturday for the fashion seminar. She goes with Ross but there is no sign of Mark. However, only a brief scene is shown at the seminar. As Ross insisted on going, Mark may have sat with someone else he knows.
When Ross and Rachel get back to the apartment and have their argument the door into Monica's room is wide open and it appeared to be devoid of furniture, most notably Monica's bed.
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.
Just after Chandler says "I'm the smallest person in the world", and Joey comes out of his room, you can see a boom mic track Matt LeBlanc across the set, and the bottom of some stage lights in the middle.