Other mistake: When Phoebe and Ross are at JFK, some of the people who are at that airport are the same people who are also at Newark. It's particularly funny to see the people behind Ross and Phoebe at JFK when Phoebe claims they're on their honeymoon, and when it cuts to Rachel at Newark as she's looking for her boarding pass, those same people are there too.
Other mistake: At the beginning of the episode, Abby is almost run over by a man driving an SUV. They exchange words and then Abby throws a supposedly full cup of coffee at him. The cup of coffee bounces off the seat of the man's car and it is obvious that the cup is empty.
In the Shadow of Z'ha'dum - S2-E16
Other mistake: In this episode, a simple light frequency modulation enables Sheridan to briefly see the Shadow creatures. But because J. Michael Straczynski decided that this was just "too easy," the technique was "accidentally on purpose" forgotten about for subsequent episodes. If the Shadows had somehow learned to adapt and remain invisible, lines to this effect would have been written in. JMS deliberately chose to "conveniently forget" about it instead. (00:40:00)
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)
Sins of the Fathers Chapter 8: The Ultimate Slayer - S3-E8
Other mistake: When Kingpin is berating Smythe over his failure to kill Spider-Man, the camera footage Kingpin is showing of Spider-Man is merely clips from previous episodes.
Other mistake: When Century is pressing the button on the control panel to restore gravity to the vessel, you can see his finger doesn't actually touch the button. (00:18:06)
Other mistake: The establishing shots of Buy The Book make it clear that there is an optometrist next to Buy The Book, but in the season three episode premiere Shake Rattle and Rubble, a major plot point is that there is a dry cleaners next to the store, which is not only wrong but impossible.