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I didn't pay attention to it, but it is an obvious shutdown 
race.  I'll fix it.  Tim Stone (the OP) is a contributor to the 
spambayes project, and it turns out this is damned important 
in spambayes, because it appears the spambayes code never 
calls the dumbdbm.close() method explicitly.  Something else I 
learned from the dumbdbm review is that a dumbdbm's disk 
files routinely get out of synch with each other, and rely on 
someone calling close() or __del__() to get them back in 
synch.  If a .bak file doesn't already exist, though, this 
shutdown race can prevent __del__() from synch'ing the files.

Boosted priority.