The One In Barbados, part 2 - S9-E24
Character mistake: When Monica and Mike are playing table tennis, Monica puts her hand on the table and looses the point. Mike then does the same thing. Surely Monica, of all people would notice that as it would have given her the point. We also see her looking at Mike at the time. (00:33:40 - 00:39:25)
Suggested correction: Not really a mistake to not notice it, especially when it may be what a character like Monica would notice. She is being extremely competitive and frustrated because Mike just denied her a point, so it's fair to say human nature plays a part here.
The One In Barbados, part 2 - S9-E24
Character mistake: When Mike and Monica are playing ping pong, Mike's service was a fault as he released and hit the ball over the top of the table rather than behind. He then (on the same point) claims to win the point because Monica placed her hand on the table. (00:33:00)
