The heating just stank of comp wash fluid and it always concerned me that the carbon monoxide levels might be quite high in the air from it.
The heater pipes came down from the engines to a 90-degree elbow with a valve on it - this has been known to fracture and blow off, leaving a ton of shredded soundproofing and a really loud noise that you can't turn off! I know one crew that came close to crashing when it happened to them at night and they didn't know what had gone wrong.