I have done similar thing to myself in Python, sabotaging my own little project. The following is not a rare thing. Imagine this were initializing random key in TLS... But since Python syntax is arguably cleaner and more readable, it's easier to catch. But that's a bet we have to put up with...

    >>> def f(a,b=[]):
    ...     b.append(a)
    ...     return b
    ...
    >>> print f(1)
    [1]
    >>> print f(1)
    [1, 1]
    >>> print f(1)
    [1, 1, 1]


    >>> import random
    >>> def key():
    ...     return random.randint(1,10000)
    ...

    >>> def f2(a, b=key):
    ...     return b()
    ...
    >>> def f3(a, b=key()):
    ....    return b
    >>> f2(1)
    3974
    >>> f2(1)
    8684
    >>> f3(1)
    2867
    >>> f3(1)
    2867