Memory leaks: - Player logs off: Javas GC can't remove it cause your list/set/whatever still contains it. So you still have the whole instance of the player (with the IP, the real name, the display name, the list name, the health, the potion effects, ...) even if minecraft tries to save it to disc and unloads it from ram. But it can go more worse: - Multiverse (or some other multi world plugin) unloads the world the (maybe since two or more days... but still in your list...) logged off player was in. The GC can't remove it cause there's still that player instance in your map, which is holding chunks loaded, which prevents the world from unloading... This taken from a post by V10lator somewhere, but I figured I'd post it here.