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 mistake in show generally - chronological order
Starring: Stephen Furst, Mira Furlan, Richard Biggs, Andreas Katsulas, Bill Mumy, Jerry Doyle
Captain John Sheridan: If more of our so-called leaders would walk the same streets as the people who voted them in, live in the same buildings, eat the same food instead of hiding behind glass and steel and bodyguards, maybe we'd get better leadership and a little more concern for the future.
Trivia: Walter Koenig's recurring Psi Corps character, Bester, was named in honor of science fiction writer Alfred Bester, who coined the term "psi cop" in one of his many novels about psychic manipulation.
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.