At the end, when Ross and Janice are talking on the couch in Central Perk, Ross is gesturing with his right hand holding a cup of coffee with his left hand. The shot changes and he is holding the cup with his right hand while gesturing with the other.
During the end credits when Ross and Chandler are talking, Chandler's sitting with his left ankle on his right knee. In the next shot of him, his legs are reversed. Right before he gets up to give Ross money, his left leg is laying on his right one.
In the teaser, Joey is eating pizza with his hands apart on the edges of the pizza box. When the camera angle changes and Rachel says, "Is he romantic with her?" both of Joey's hands are holding a slice of pizza.
When Joey takes the 'isolation booths' off the chick and the duck, you can see the edge of the set on the right under the Foosball table.
Doug should know wearing a sweater while playing tennis would only add to his exhaustion.