Bug 629097 describes a thread race involving 
asyncore.pool(), and was closed with "so don't do that".

Polling exception sets is new in 2.4, though, and opens 
the same race to formerly safe uses:  leaving 
indeterminate junk in the socket map, provided that junk 
doesn't call itself readable or writable.  Such junk never 
got passed to select() before 2.4, but always gets 
passed to select() in 2.4.  This is incompatible behavior, 
albeit in a grey area.

More discussion starting at

http://mail.python.org/pipermail/python-dev/2004-
August/047817.html