Continuity mistake: The prom video has just ended and the TV screen goes blank. Monica had been sitting next to Rachel, but in the reflection of the TV screen you can see her beginning to stand up. We cut back to a 'real' view, and she's sitting down; then she stands up again.
Continuity mistake: On the actual prom video tape, it only takes Ross a mere 12-13 seconds to run up the stairs, completely change outfits into his Dad's tux, and reappear ready-to-go on the stairway. This is impossibly fast. Also, it is not possible that the prom video tape was stopped and re-started during this time, which could account for the short duration. The reason is because Ross's mom was the one filming at this point. It turns out that she has no idea how to stop the video recorder. Thus, the video tape was one continuous recording, and we are expected to believe that Ross could change outfits within 12 seconds.