To clarify, what is actually happening is that when a member of a group of windows (expanded by the decombine active items option) is middle clicked, the group collapses, specifically on middle mouse down. It collapses fast enough that, even with a quick tap, when the middle mouse is released, it closes the tab that is now underneath the pointer.(or whatever else middle mouse is set to do)

It is extremely obvious with a vertical task bar as the space between items is so small. With a horizontal task bar it only happens if you mclick the far end of the last item in the group.
(using windows 7 64, if that helps)