That happens all the time. Sometimes Imps drop Treasure Boxes, Spells and Traps and nothing happens. Treasure Boxes may be used sometimes but the Spells and Traps stay there forever.
Sometimes those nasty little Imps keep carrying traps forever, can't pick them up or anything.
Nowadays I always save before I make an imp pick up an item. Too much risk.
Oh yeh, might be related to the thing limit. Or the entire thing thing freaking out. It's not the actual thing limit though. Other Imps will still place Traps for example.
Must be a leak in the code somewhere, that the a.i. forgets that those things are there or that they are carrying that object.