I'm investigating a memory leak in qBittorrent, and it appears that MooTools is the culprit.

The issue is in clean, which stores item.uniqueNumber in uid, then calls item.removeEvents, then checks if uid != null to decide whether to clean up the item's storage.

The problem is that item.removeEvents can call item.retrieve('events') which always calls Slick.uidOf and get, which assign item.uniqueNumber and allocate it storage respectively.

The memory leak can be resolved by either preventing retrieve from allocating storage (since read operations shouldn't affect state):

	retrieve: function(property, dflt){
-		var storage = get(Slick.uidOf(this)), prop = storage[property];
+		if (!this.uniqueNumber || !storage[this.uniqueNumber]) return;
+		var storage = storage[this.uniqueNumber], prop = storage[property];
		if (dflt != null && prop == null) prop = storage[property] = dflt;
		return prop != null ? prop : null;
	},

or by taking item.uniqueNumber in clean after calling item.removeEvents:

var clean = function(item){
-	var uid = item.uniqueNumber;
	if (item.removeEvents) item.removeEvents();
	if (item.clearAttributes) item.clearAttributes();
+	var uid = item.uniqueNumber;
	if (uid != null){
		delete collected[uid];
		delete storage[uid];
	}
	return item;
};

I'm happy to create a PR for this.