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Well, try making an "empty" class Foo(object): pass, and see
what magically shows up in Foo.__dict__.keys().  Here is the
__dict__ descriptor used for instances of Foo.  This
descriptor is connected to subtype_dict() and
subtype_setdict() in typeobject.c.

INCREF/DECREFs are in theory missing around every use of the
dictionary returned by _PyObject_GetDictPtr(), because more
or less any such use could remove the dict from the object
from which _PyObject_GetDictPtr() returned from, and so
decref the dict - while it's used.  This might require some
timing, to check the performance impact.