The One With Phoebe's Cookies - S7-E3
Visible crew/equipment: When Chandler and Jack, both wearing robes, are walking toward the whirlpool room, just as Chandler tells Jack that he'd feel more comfortable if Jack didn't tell anyone what had happened, we can see the shadow of the boom mic moving across the wall at the right side of the screen.
Super Grover
The One With Phoebe's Cookies - S7-E3
Other mistake: When Monica realizes that the cookies were the Nestle Toll House recipe, Monica tosses the yellow bag of Nestle brand chocolate chips to Phoebe, and the brand name has been partially covered so it looks like "I Iest" and the Toll House logo on the side has also been partially covered.
Super Grover 
The One With Phoebe's Cookies - S7-E3
Continuity mistake: While trying out different recipes, at one point Monica's holding a cookie in her hand. It's a whole cookie, but when the angle changes, suddenly a chunk is missing. (00:13:00)
Jon Sandys The One With Phoebe's Cookies - S7-E3
Visible crew/equipment: When Ross asks what do with all the cookies, a boom mic can be seen at the top of the frame. (Full-screen version only).
Lightrock
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.