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.
Babylon 5 (1994)
1 deliberate mistake - chronological order
Starring: Stephen Furst, Mira Furlan, Richard Biggs, Andreas Katsulas, Bill Mumy, Jerry Doyle
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)
Susan Ivanova: Vakar Ashok, our gun arrays are now fixed on your ship and will fire the instant you come into range. You will find their power quite impressive... for a few seconds.
More trivia for Babylon 5
Question: What did Susan do after she left at the end of season 4?
Answer: She was promoted to Captain and headed a new class of ship called the Warlock. Hence she was no longer on the station after season 4, dealing with her grief from Marcus dying.
Chosen answer: She was promoted to Captain and thus had to leave Babylon 5, becoming a captain on a new Warlock class ship. Later on she retires/quits and becomes the top Ranger on Minbar.