In the game Taskmaker, one of the things I always try to do as soon as possible is acquire a vapor ring (allows you to walk through nearly any wall/tile). Facing a bad guy who is too strong? Become ethereal and walk into the middle of a wall, rest up, and return until he's beaten (or you run out of food or get bored).
Not to mention all the looting potential.
When I originally played the game, it was a demo that didn't include most of the locations. It did include a range of items. So, I knew this trick before I got the full game. I had practically become the taskmaker before I had the full game.
Ghosts pass through solid wall/door. Celebrate finding a key. Unfortunately they cannot remove it from the room as it is solid and cannot pass through matter like they can. Ghosts get depressed about this.
The ghost is sad because it locked itself out/forgot the key inside. Second ghost shows up an tells them to just warp through because they are ghosts. But instead of being happy they are inside and never needed the key, they try to move the key outside so they can unlock the door to get inside.