Rachel says that she and Ross went out for two years when they all sit down for dinner. In actuality,they were together from February 1996-February 1997. They did get back together for a little bit, but officially, it was only one year.
At the very end, when Joey comes back wearing his "Thanksgivin' pants," he pulls the chair away from the table twice before sitting down.
While Ross is telling everybody the story about making out with the librarian, in the mirror behind him the red table cloth disappears and reappears in every shot.
When Rachel walks in to the apartment, Will has his hand on the flower display on the wall and he crushes it. However, in the next shot the flower display appears untouched, then the next shot his hand is on the display crushing it again.
Chandler says he's only pretending to watch football in order to get out of helping and says he really has no idea what's going on. However, in a previous Thanksgiving episode "The One with the Football" he is shown watching and really being into the game. He also plays football and knows all the rules of the game.
When Will talks about his weight loss, he says he avoids complex carbohydrates because they are his worst enemy. Complex carbohydrates do not contribute to weight gain if eaten in moderation - only simple carbohydrates cause weight gain because they are digested more quickly and lead to feeling hungry sooner. What's more,, yams are not complex carbohydrates.
When Ross was talking about the librarian the mirror that is behind him shows his reflection and that of a wood floor in between the camera being on him and then back again.
While Monica says "everyone in our school heard it!", you can see Chandler behind her saying his line "everybody in my school heard it!" but there is no sound coming off of him and when the shot changes to him, he says his line from the start.
Monica's mouth does not move when she says Rachel's name when telling her and Will where to sit at the dinner table.
It is Thanksgiving Day -- Rachel conceived in April (known because Monica & Chandler's wedding was booked for May) -- therefore, over 7 months have passed. Rachel should be due sometime in January and "showing" much more than she is now.
At one point when watching the football game, Chandler says to Phoebe that "Anderson" scored a touchdown and then says to Phoebe that there is no Anderson. In reality, the Detroit Lions had a wide receiver named Scottie Anderson during that game, and he caught their last touchdown (Detroit lost 29-27).
Monica could've just cooked a small turkey for Thanksgiving since she, Joey and Ross are the only ones who would eat it. Monica says it's about 19 pounds. According to Google, tom turkeys can be as small as 16 lbs, and hen turkeys start at 8, Monica could definitely have found something smaller.