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)
Six Forgotten Warriors Chapter 4: The Six Fight Again - S5-E5
Other mistake: The news footage of the elderly Six American Heroes being attacked by the Insidious Six is merely clips from the previous episode.
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)