When Chandler opens the door to the bathroom to reveal Joey asleep, the toilet is to the left of the door. However, in earlier episodes, the toilet has been shown as being to the right.
At the coffee house, Rachel claims that Chandler's old roommate Kip dated Monica and when they broke up he was "phased out." However, in an earlier episode, Chandler tells Joey that he and his old roommate Kip bought a Hibachi together and then Kip "ran off and got married".
Joey mentions the hotel called to say they found an eyelash curler. That could not have happened. Monica never unpacked. She stormed out with her suitcase after they had the fight.
When Joey figures out the truth about Monica and Chandler and Chandler drags him into Monica's bedroom, Monica is right behind them. However, in the next shot, she is clearly three or four seconds behind them.
When Joey figures out the truth about Monica and Chandler and Chandler drags him into Monica's bedroom, the artwork and sconce on the bedroom wall disappear and reappear between shots.
Chandler & Monica go to Vermont for the weekend. During the whole "room switching" fiasco, Monica says, "this is a garden view room and we paid for an ocean view room". Quite a trick as Vermont is nowhere close to the ocean.
When Chandler and Monica are at the hotel, Monica keeps insisting they change rooms. Chandler commented that they changed rooms 10 times. But back at home, when Monica tells Chandler she wants to pay for half the room, Chandler says $300 and says "Just think of it as $20 per room". - Chandler says "$25 per room," so taking into account the first room as well as the 10 room changes, bringing the total number of rooms up to 11, he could simply be rounding down from the $27 and change it would actually be per room in his heatedly sarcastic estimation to Monica.
When Chandler gets excited on the bed when he sees the car chase on TV, the blankets on the bed get pulled up and you can see there are no sheets on the bed.
With the way Monica is portrayed about obsessively cleaning, it is unlikely she would fail to notice a large box newly placed under her bed (by Joey).
At around 18:04 when Joey walks in and says "Hey, Rach", his mouth is clearly only saying "Hey" when we hear Rach(el).
When Ross is trying to tell Rachel that Emily won't let him see her anymore, Rachel is reading a letter from her mother and finding out that her family's dog died. Rachel's mother lives on Long Island, and it seems very unlikely that she would be sending Rachel a letter rather than picking up the phone and calling her.
Chandler and Monica are supposed to be away for the weekend. When they finally find a room that satisfies Monica and Chandler is searching for the car chase on television, the theme from ER (1994) is playing. Chandler mentions that it is regularly scheduled programming. ER was part of NBC's Thursday night "Must See TV" lineup.
When Ross finally manages to tell Rachel about Emily's request to not see her any more, she says 'what are you going to tell her?' followed by 'you already agreed to this, didn't you?' but she was with Ross when he was on the phone to Emily saying 'that thing you wanted me to do, I can do it' after she told him to give her whatever she wanted, meaning she already knew he'd said 'yes'.