The One With The Hypnosis Tape - S3-E18
Factual error: When Pete takes Monica to Rome for the date this would have been impossible as Rome is 6 hours ahead of New York and at least 4/5 hours travel time, assuming they went by Concorde (double that otherwise). They would have had to have left New York early in the morning to make dinner for 8/9 pm in Rome.
Factual error: In the scene where they are gathered to watch the episode of Songs of Praise in the shot of Alice's reading. Her quote 'from the sixth chapter of the Song of Solomon, beginning at the second verse' "ye are the fault [salt] of the earth and fainted [sainted]." doesn't exist.