In Tow The Pediatrician, Ross and Rachel come back from Dr Gittleman's office and arrive at Monica and Chandler's apartment.

As they walk in, we hear Monica say "How was it?" and we see Rachel and Ross walk through the door. Rachel says "It was great, we really like him". During the first part of the sentence we see Ross and Rachel walk through the door, but as Rachel says "We like him" there is a close up of her and Ross and her lips don't even move as she's saying those words.

Very noticeable if you look for it.