weakref to a dead object, or via any Python code run in any other thread
that managed to obtain the GIL while a __del__ or callback was running
in the thread doing gc). The most likely symptom was "impossible"
- ``AttributeEror`` exceptions, appearing seemingly at random, on weakly
+ ``AttributeError`` exceptions, appearing seemingly at random, on weakly
referenced objects. The cure was to clear all weakrefs to unreachable
objects before allowing any callbacks to run.