The One With Phoebe's Wedding - S10-E12
Continuity mistake: When Monica and Phoebe are talking about having a harpist versus Marjorie playing the steel drums, the handbag strap over Monica's shoulder vanishes and reappears.
The One With Phoebe's Wedding - S10-E12
Revealing mistake: Just before we see everyone setting the wedding up outside, there is a sweep shot of the city with snow falling. It's very obvious that this snow is added digitally and it's very badly done. There is no snow on any of the buildings, and there is no snow hitting the surface of the water. (00:20:50)
The One With Phoebe's Wedding - S10-E12
Revealing mistake: At the wedding, none of the snow melts when it has been sitting on people for several minutes. Occasionally when it gets brushed off clothes it just sticks to hands and sleeves, again for several minutes without melting. (00:21:15)
The One With Phoebe's Wedding - S10-E12
Continuity mistake: After Phoebe tells Monica that she is fired from being her wedding planner, Monica says that she has never been fired. However, in "The One With Five Steaks and an Eggplant" Monica gets fired for taking the kickbacks.
The One With Phoebe's Wedding - S10-E12
Continuity mistake: After Ross greets Phoebe at the rehearsal dinner, he moves toward Mike as if he is going to speak to him. The camera cuts to Phoebe and Ross is now standing behind her, facing away from where Mike is.
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.