This has happened to me if the user clicks on the tree control while my code is programmatically updating it (typically removing an item). It doesn't happen often, because the timing has to be JUST right to produce the problem.
This is caused by a good old-fashioned bug in the version of COMCTL32.DLL shipped up to and including IE4: But I can't get Microsoft to admit it, despite being able to repro it with a tiny little test program <sigh>. Installing IE5 fixes the problem, because this provides a newer fixed version of the control DLL.
I doubt this will trouble anyone anymore.