I have seen XP spirits swirl all around, move around up and down blocks, like they are trying to get to me, but just circulate, even with me standing there. Ihave seen sprites do this as well. One time, I saw a repeating Rotten Flesh sprite fall off a cliff (~20blocks away) every few seconds, until I finally went to that position, and they vanished. Sometimes this issue is annoying, especially after a large killing spree in one area...everything just swirls around the landscape, but doesn't get collected.