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.
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.
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.