As the six friends are telling stories about the past Thanksgivings, they reveal Joey got his head stuck into a turkey back in 1992, while in The One with the Flashback (1996) it is revealed that Joey moved in Chandler's apartment in 1993, meeting the rest of the friends (except Rachel) then.
Monica has previously stated that when she first met Chandler she thought he was gay, but during this flashback, it is revealed that she had a crush on him when she first met him at their family's dinner.
At the start of the episode, when Phoebe tries to turn the TV on, the remote switches positions at least two times between shots.
In "The One With All the Thanksgivings," when Chandler is in the hospital, the doctor goes from wearing gloves to not wearing gloves in consecutive shots.
When Monica has the turkey on her head, the fez not only goes from being on top of the turkey to behind the turkey without anyone moving it but it also seems to defy gravity as it is almost vertical to the turkey but it doesn't fall off.
It is unlikely that Monica would have another fully defrosted turkey to put on her head, immediately after they ate thanksgiving dinner.
In Phoebe's flashback to the Civil War, the tent she is in is green and she wears a while apron. During the Civil War (and until WWI) tents were made of white canvas and aprons were brown (so as not to show dried blood).
In the flashback to 1992 (before the show started), the bathroom floor is black and white checkered and the closet door has the handle on the right. At that time, the floor would have been all white and the handle would have been on the left.
In Pheobes flashbacks, the arm that gets "blown off" is clearly fake, and the outline of her real arm is seen tucked into her clothing.
It is impossible for Rachel to "give her flower to Chip" because Monica had told Rachel's father that she had sex on his bed during her freshman year.
During Chandler's flashback to Thanksgiving with his parents, they are clearly discussing the divorce over the main course, with everyone still eating turkey. This does not correspond at all to Chandler's statement back in Season 1 that his parents first told him they were divorcing just as he had eaten "a mouthful of pumpkin pie".