The One Where Monica and Richard Are Just Friends - S3-E13
Continuity mistake: When Richard sits in Monica's room, when Monica opens the door we can see in the background the living room's window draperies, table, picture frame, etc., but they're not as they should be. In this mock-up the shade is pulled down, the draperies look different, and the direction they're all facing differs. (00:15:55)
The One Where Monica and Richard Are Just Friends - S3-E13
Continuity mistake: Monica and Richard are squishing tomatoes; when she starts dabbing his shirt with club soda the door behind them leading to Monica's room goes from open to shut between two shots. (00:08:30)
The One Where Monica and Richard Are Just Friends - S3-E13
Continuity mistake: When Monica is in Richard's room she lights candles then realises Richard's home with another woman so blows out the candles but misses one. Next she hides under the duvet waiting for Richard. When he comes in all candles are unlit but she is still under the bed. (00:18:00)
The One Where Monica and Richard Are Just Friends - S3-E13
Continuity mistake: When Monica is dabbing Richard's shirt, the way she is holding the cloth changes between shots. Note the amount over her fingers. (00:08:50)
Answer: Usually by the seventh night of Hanukkah you don't know what to get the other person, and you will probably resort to getting the other person something insignificant. Chandler is basically saying that the pen is a stupid gift.