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)
The Return of Hydro-Man: Part 1 - S5-E7
Other mistake: When Spider-Man breaks the chain off the steering wheel of the boat, several of the links on the back loop of the chain break off, which would be impossible since he broke the chain at the front.
The Grim Reaper Wears a Teflon Coat - S1-E5
Other mistake: For the first few episodes of this show, Rhodey is voiced by James Avery. However, there are two scenes in this episode with Rhodey providing voiceover, and you can tell he is voiced by Jim Cummings. Presumably Avery wasn't available to record the dialogue for these parts, so they just went with Cummings instead. (00:10:36 - 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.