Sorry for the earlier noise.  I'm fighting a flu and my head is mush :-(

Anyway, this doesn't look obvious.  We get to this point:

            if (Py_REFCNT(op) == 1) {
                /* op will be destroyed */
                gc = gc->gc.gc_prev;
            }

and op is the type object for class B.  gc gets set to the previous object, a list.  Everything looks fine at this point.  But when we get back from:

            Py_DECREF(op);

the list's gc.gc_next field has been overwritten with NULL.  That's why gc gets set to NULL on the next trip through the loop.

I spaced out stepping through all the type deallocation code, and didn't find exactly when the list's gc_next is overwritten.  The list's gc_prev is still fine.  Perhaps some code called _PyObject_GC_UNTRACK on the list object (which NULLs out the gc_next pointer but not the gc_prev pointer).