I was milking the trigger that spawns ghosts and apparently, after spawning too many, the game decided to place one outside the map.
Also, after killing most of 'em, the game couldn't find a spot to drop the ectoplasm, giving a big warning on the console. I guess it would be better to combine the contents of fixed drops instead of creating new bags.
During this fight I also found out that imbuing demon oil makes it no longer an area of effect weapon!