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)
Continuity mistake: The orientation of the planes on the aircraft carrier at the beginning of the episode changes between shots. (00:01:00)
The Origin of the Fantastic Four: Part 1 - S1-E1
Other mistake: When Puppet Master is making a clay figurine of Thing, the mold he uses doesn't have a Thing-shaped interior with which to make a detailed figurine. The figurine should have just looked like an undefined human-shaped figure. (00:15:20)
Partners in Danger Chapter 5: Partners - S4-E5
Other mistake: When baby Silvermane is secured to the platform, his arms are long enough to reach inside the restraints. Considering he is physically a baby, the mistake is that the animators either goofed and drew him to be a little older, or that his arms are disproportionately long.