Double-clicking on the triangle that expands a thread in the thread pane (in either the mail or news context) will open up the currently selected message (which is not necessarily the same message that the thread triangle belongs to) in its own message window.
I can't reproduce this, clicking/doubleclicking on twisties does nothing but toggling the open/closed state of the twisty for me.
I'm able to reproduce this on the 5-11 win98 build. I haven't tried todays.
I see this in June 22 build. Annoying.
Build 2001-05-20-04: WinMe, I see it also.
I have a bug related to this: When you double-click on a threaded message, it both opens the message and expand thread. Should do either action, not both.
*** This bug has been marked as a duplicate of 92194 ***