I've noticed occasionally with javascript tooltip scripts that sometimes the tooltip does not get hidden when you mouseout of the element which causes the tooltip to appear. This only happens when the cursor is moved over the element very fast, and even then rarely happens. The problem seems to be agitated with tooltips which are rich with html/images, as well as dynamically fetched tooltip content(ajax or just appending a new script element to the head to load in the data). Having the computer run other cpu taxing programs seems to agitate this as well. Again, this is very rare but it happens now and then.

Since this problem is present in IE, Firefox, and Opera, and I've noticed it on different computers, I tend to think its somehow related to available cpu cycles. I've always imagined that the mouse hand/cursor position is handled by the operating system, and other programs/windows probably poll/read the cursor position from the os somehow. This would explain why you can usually still smoothly move your cursor over a program that has become unresponsive or is terribly lagged out.

So do you think maybe if the program is so lagged out(the web browser in this case) that its time between polling the os for the mouse position increases, causing it to miss/not fire the onmousout event?

I know, odd question