Oh oh.  It's that time of year again.  I'm sure the 
attached (temp2a.py) can be simplified, perhaps down 
to two objects and one thread.  As is, I *think* it 
demonstrates that invoking a weakref callback can do 
fatal damage, not necessarily because of what the 
callback does, but because simply calling it while gc is 
running can allow other threads to run during gc too, 
and resurrect a piece of cyclic trash T that's already 
been tp_clear()'ed, via invoking a still-living weakref to 
T.

If so, this isn't new in 2.4.  It's a real problem since 
temp2a.py is what's left of ZODB 3.4a1 <wink>.