The One With the Sharks - S9-E4
Revealing mistake: While Monica and Rachel are talking about Chandler's "jollies to Jaws" Rachel's wearing a colorful flowery top, and after Joey walks in just as Monica takes her seat at the table in the closeup, it's Jennifer Aniston's stand-in, wearing a blue top, who is seated beside Monica. Rachel's hair is wavy in the flowered top, but straight in the blue top (viewed in widescreen). (00:12:22)
Visible crew/equipment: When the son of the accident victims is being led through the trauma rooms to see his parents' bodies, you can see Laura Innes crouching under a table in the background directing the scene. Innes was the director of this episode and it was established that this episode took place on her character's day off. Therefore, she should not have appeared in front of the camera at all. (00:32:30)
Behold, a Distant Star - S2-E10
Character mistake: When Sue is brought out of surgery, her father can be seen wearing a watch. Surgeons don't wear watches or any other kind of jewelry when performing surgery, as they can't be sterilized and can also get in the way of gloves. (00:05:00)
Factual error: Several episodes depict the moon in the night sky as being far too large.
And the Sea Shall Give Up Its Dead - S1-E1
Other mistake: When Tony Stark is typing on the keyboard, some of the keys are depressing without him touching them.