Using dumbdbm for simple testing, I occasionally get the following 
error:

Exception exceptions.AttributeError: "'NoneType' 
object has no attribute 'error'
" in <bound method 
_Database.__del__ of <dumbdbm._Database instance at 
0x008B356
8>> ignored

I've tracked it down to a 
problem with the releasing of the reference to _os during __del__, 
which then calls _commit.  _commit proceeds to use the _os, 
which is given a value of None... blamo...

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Tim, did you see this during your review of dumbdbm?

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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.

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Fixed, in

Lib/dumbdbm.py; rev 1.26
Misc/NEWS; rev 1.810