When Chandler and Rachel are watching the Thanksgiving Parade, Rachel gets up to go take a nap. When the camera pans, the TV is turned off; when the camera pans again, the TV is on.
When Rachel is showing Ross all the things she has kept and pulls out the dinosaur nail, it switches back and forth from the pointed end sticking out to the darkened end. This happens without Rachel actually moving the nail.
When Joey walks up to the counter at Central Perk, there's a potato in the cake tray in front of Gunther. However, this may not be a Goof; it could just be a practical joke by Gunther or other employee.
When Monica and Tim are on the balcony, the edge of the set wall is visible through the window on the left-hand side.
While Chandler is in the box and Kathy is speaking to Rachel and Phoebe for the reasons Chandler is in the box while the camera switches between them, every time the camera is on Kathy, the edge of the set and equipment from the stage can be seen.
When Chandler is in the box and Ross 'accidentally' puts his hand over the air hole, he places it near the hole first, moves it over the hole, and then sort of glances towards the box to check it is in the right place.
Ross (and everyone else) is surprised that Rachel exchanges gifts, however it is established before this that Rachel exchanges gifts and in Season 2, in The One with the Two Parties (1996), Ross tells Rachel she can exchange the earrings if she wants.