http://bugs.winehq.org/show_bug.cgi?id=26543
--- Comment #7 from Nikolay Sivov bunglehead@gmail.com 2011-03-24 12:11:58 CDT --- Ok, this is kind of suspicious:
--- trace:monthcal:MONTHCAL_WindowProc hwnd=(nil) msg=1002 wparam=0 lparam=71ad5fc trace:monthcal:MONTHCAL_WindowProc hwnd=(nil) msg=1009 wparam=0 lparam=32ead4 trace:monthcal:MONTHCAL_WindowProc hwnd=(nil) msg=46 wparam=0 lparam=32ea3c ---
I think this can happen only if application subclasses Monthcal and for some reason sends null window handle.