I've also had this problem! I believe I've experienced it since the Hispanic heritage patch. Placing wall decor on angeled walls gives the error message "Must be placed on a room wall", while the item is clearly placed on a wall on which it should fit. I've also had issues in placing items on the floor if they're at an angeled wall if the items is angeled as well, making sims not able to use the bed or whatever it is. Placing with bb.moveobjects on fixes the problem temporarily, but it's still a problem, with objects placed on angeled walls sometimes dissapearing if I place a lot or a room.
