Corrected entry: When Phoebe and Chandler are about to kiss there is a bottle of wine and two glasses on the counter. It then cuts to a view of Phoebe with no bottle or glasses on the counter, just a phone, and then later in the same scene it cuts back to Phoebe again and there are two glasses and a bottle of wine again.
Corrected entry: When Phoebe and Joey are arguing about Phoebe's guitar teaching lessons, Monica comes in between them and gets her picture taken with them. Who takes the picture? Monica, Joey and Phoebe are all in the picture, Ross is on his date, Rachel is out (she comes in just after the picture is taken) and Chandler isn't there either (he is on the phone to Monica when Rachel picks up the phone). So who did it? There is no furniture there for Monica to have put it high enough to get them all in if she'd set the timer, either.
Corrected entry: In the scene where Phoebe is describing her worst Thanksgiving experience (in 1862 as a nurse) and her left hand is blown away, you can see that the hand is fake as it doesn't move, is shorter and is a different color to the right one. Additionally, in the time between the bomb exploding and her hand being torn away, a rope is visible moving it.
Corrected entry: Monica tells Chandler she is not going to go on the date with the man nurse. As she is walking away, Chandler is going to do a dance and she says don't do the dance. The dance she is referring to is the one he does where he hears that Monica is with someone who is the best sex she's ever had. How can she know about this dance if the first time she sees it isn't until the following episode?
Corrected entry: Phoebe is collecting for charity and Monica comes to see her; a man puts a dollar in the bucket and tries to make change for the bus. When Phoebe argues with the man he says "bite me blondie", but he couldn't have known that she is blonde because she has a Santa hat on that covers her entire head and no hair is coming out from underneath the hat.
Corrected entry: When Phoebe is talking to her dad in the coffee house, she asks Frank Sr. about his marriage to Lily and their two children. But Phoebe has already found out that Lily isn't her real mother because she lives in Montauk. Frank Sr. would surely know that Lily wasn't there real mother if indeed she wasn't.
Corrected entry: In The One With The Ball, Monica ends up showing she has an allergy to cat dander. But in the previous season, in The One With The Cat, Phoebe finds a cat that she thinks is the reincarnated spirit of her dead mother. Monica is in close contact with the cat, and even petting it in one scene. Now, it's possible to develop allergies over time, but the time between these two events is very short (in storyline time).