Jinkies! Based on a submission from @thesealfriend
There are two ways this could have happened:
1: The players fought their way through a dungeon filled with undead and just forgot ghosts were a thing.
or
2: The players thought they were being super clever and setting a bunch of traps for whatever the DM had waiting for them, and said DM decided to 'Gotcha!' them by spawning in a few incorporeals.
If I had to bet, I'd put my money on scenario 2. This idea that the players have to 'beat' the DM and the DM has to 'beat' the players is the Catholic pedophilia of the gaming world. Everyone laments that it's so horrible and just needs to stop happening, but the very system underpinning it continues to give rise to the same problem over and over.
Whenever people recount their adventures to me, my response is usually something like "Cool, cool, and how did that relate to the greater story of the game?" and they always give me blank looks. There is no story to D&D. It's just a series of Gotchas and Zingers and Burns so people can pretend, if only for a moment, that they're better than someone else.
