Run the following script with Python 3.6.0rc1:

    class O:
        pass

    o = O()
    for n in range(20):
        print(n)
        o.x = 42
        o.__dict__.pop('x', None)

You can observe the memory usage of the Python process growing exponentially.

E.g. in bash:

    ulimit -v 1000000    # don't push other processes into swap please
    python3.6 break.py
    0
    1
    2
    3
    4
    5
    6
    7
    8
    9
    10
    11
    Traceback (most recent call last):
      File "break.py", line 7, in <module>
        o.x = 42
    MemoryError

If you're curious where this happens in real life, py.test's CaptureManager.deactivate_funcargs() does

    self.__dict__.pop("_capfuncarg", None)

and I found it by running 'tox -e py36' on https://github.com/mgedmin/eazysvn

This is the same problem as #28147.