When Ross and Rachel are on the plane and Rachel takes off her cardigan she has taken it off her shoulders in the first shot, but in the next shot she is just starting to take it off her shoulders again.
When Chandler comes down looking for Monica, she's seen playing at a table nearby. She hugs a man next to her, but in the next shot she hasn't hug him yet. Next shot, she still hugging him and in the very next shot, she's approaching to hug him.
Chandler comes down to find Monica and sees that she's playing some game and having fun. He makes a sad and dejected face. The scene cuts back to Monica, and then to the exact same shot again of Chandler making the sad and dejected face.
When Chandler leaves the hotel room to go find Monica to make up after their fight Phoebe picks up the gladiator helmet to try it on. As she moves over to the mirror the end wall of the fake hotel room and hallway is visible.
When Ross is leaving Rachel's, a stagelight can be seen by the bathroom.