The following example crashes:

class TypeVar:
    @property
    def __module__(self):
        1/0

str | TypeVar()


Output:
Fatal Python error: _Py_CheckSlotResult: Slot | of type type succeeded with an exception set
Python runtime state: initialized
Traceback (most recent call last):
  File "<stdin>", line 4, in __module__
ZeroDivisionError: division by zero
Aborted (core dumped)

The problem in Objects/unionobject.c is that is_typing_module() (and therefore is_typevar() and is_special_form()) can return not only 0 and 1, but -1 as a signal of error, but is_unionable() does not check results for error and interprets it as boolean true.