When making out with the Dirty Girl, Ross's hand touches some sticky black liquid (probably chocolate) that covers his hand. The next second his hand is clean again.
Two different high-rise apartment buildings are used for the exterior of Mrs. Burkart's apartment where Monica and Phoebe have just catered the wake and are attempting to get paid.
When Phoebe offers to loan Monica money to buy catering supplies, the two hug and then Joey comes in with Cathy. The next shot shows Phoebe sitting down and Monica walking from the kitchen toward her bedroom with the phone. They wouldn't have had enough time to move that quickly.
When Chandler asks Rachel if she thinks he is being pathetic about Kathy's birthday present, Rachel says: "Ooo, pathetic!" but Jennifer Aniston's lips mouth different words.
When the chips bag starts rustling and Ross (thinking it's a rat) hits it with a magazine, a crew member is visible lying under the coffee table, moving the chips bag with his hand.
Boom mic comes into frame for a second during funeral scene, after Monica places cards on table.