Deliberate mistake: Though he moved freely about the station in his environmental suit, Kosh was never actually seen going through a door. He approached doors, but then, sometimes rather disconcertingly, would suddenly be shown on the other side. The reason: look closely and you'll realise his costume was too large to fit through any of the doorways on the set, so in reality the character would never have been able to move around the station.
Visible crew/equipment: When the Ikarran tosses Sinclair into a pile of cargo crates, the commander's stunt double's face can be seen briefly. (00:33:05)
The Parliament of Dreams - S1-E6
Revealing mistake: As Lennier is first setting foot on the station, the same extra rides the escalator in the background up twice, even though we never saw him (nor was there time for him to) come down.
And the Sky Full of Stars - S1-E9
Revealing mistake: Inside Sinclair's mind, just after the knight shoots the fake Garibaldi, he lowers his gun and the end briefly lights up red. Evidently the actor accidentally pulled the trigger on the prop and triggered the light that lets the CGI team time the effects correctly. Note that it is not the gun barrel glowing after shooting, as it is not lit up before he lowers it.
Continuity mistake: When Garibaldi is saying goodbye to Lianna her hair keeps moving from behind her shoulders to hanging over her right front shoulder. (00:40:55 - 00:43:40)
Visible crew/equipment: Approximately 29 minutes and 10 seconds into this episode, right after Sinclair clasps hands with "The One", Garibaldi and Sinclair are on the ground. In the upper right corner of the shot there is a grip with a red baseball cap and red and white shirt on. He has long hair and spiffy glasses.
Continuity mistake: When Cortez is coming through the B5 gate, you can see it open on the monitor behind the officer announcing Cortez' arrival in C&C. In the next cut you see the gate open again. (00:01:10)
A Race Through Dark Places - S2-E8
Continuity mistake: Before she's succeeded in her effort to move it with telekinesis, Talia's penny reverses itself on the table between takes, changing from tails-up to heads-up.
Continuity mistake: While she stands at attention and the Sgt. Major chews out her unit for fighting, Dodger's hair keeps recombing itself every time the shot changes angles. Her bangs are parted, then solid across her forehead, then parted again. (00:20:00)
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)
In the Shadow of Z'ha'dum - S2-E16
Continuity mistake: When you see Morden's picture on the screen before the theme title, he is smiling, but after the theme title ends, he is in a different position and is not smiling.
Audio problem: In the last scene of the episode Sheridan says to Mr. Sebastian "The morning after the last of a string of murders in the East End." His lips however, say West End instead of East End. The script and initial broadcast used the words East End, but after it was pointed out that this was factually incorrect, the line was dubbed over for all subsequent broadcasts and releases. (00:38:50)
Revealing mistake: When he's attacked in Down Below, Dr. Franklin is shoved against a station bulkhead/wall that wobbles when he hits it. (00:15:30)
The Face of the Enemy - S4-E17
Visible crew/equipment: When Sheridan is attacked in the bar on Mars, the strobe lights at one point cast the camera's shadow against one wall behind the actors.
Continuity mistake: At the security meeting, just as Captain Lochley is saying "I agree with Sheridan", she is shown in a long shot uncrossing her arms and starting to move forward. The camera goes to a close-up of her, and she once again is shown uncrossing her arms and starting to walk forward.
Continuity mistake: The ship that's supposed to transport Londo Mollari home changes ship class. Before the explosion, it's a Primus class battle cruiser. The wreckage shown afterwards is of a Vorchan class medium warship.
In the Kingdom of the Blind - S5-E9
Continuity mistake: On Centauri Prime, Londo has an audience with the demented Regent. In the initial shot, the babbling Regent holds both hands up near his face as he speaks. In the very next, reverse-angle shot, his hands are down at his sides. When it cuts back, his hands are up again. (00:36:30)
The Corps is Mother, the Corps is Father - S5-E13
Factual error: Bester lifts the blanket covering the bloody corpse of Harris' roommate. The man hasn't been dead long enough for the blood to dry, so there should be blood-transfer on the underside of the blanket - but there's not a spot on it.
Answer: According to Babylon 5 lore, PSI Cop Bester was quoted, "We learned some interesting things about Ms. Winters in the course of her debriefing and dissect-that is, examination.