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Firstly, on the matter of threads.  Tcl/Tk on Windows uses
threads internally even in non-threaded builds.  That's
Windows for you (in particular, you can get calls parachuted
in from completely separate threads with relatively little
warning.  Also, you need threaded helpers to handle
non-blocking waiting on some kinds of objects.)

It's a bit hard to work out if the problem's been fixed
without a stack-trace (preferably with debugging symbols
enabled so that we can see what the function names and
arguments to those functions are.)  I have no  chance of
replicating this bug here (I'm currently running IRIX, but
also have some access to Solaris and Linux...)