Yep. Happens with all the tooltips from time to time, but it's super rare. (btw, this is not MWO only thing. windows has been doing that since Win 3.x) if i were to guess, it happens when the element that the tooltip refers to is destroyed just after open-tooltip call is made. The tooltip shows up, but because it's parent object is no longer exising it never receives mouse-exit or close or similar calls.