In Python it's possible to redefine some builtin values that shouldn't really be changed. For example True and False can be changed in Python versions 2.7 and lower. This is fixed in Python 3 and assignment to True or False raises: "SyntaxError: assigment to keyword" but it works on Python 2.7:
Python 2.7.3 (default, Apr 20 2012, 22:39:59)
[GCC 4.6.3] on linux2
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>>> True,False = False,True #Exchange True and False
>>> id(1==1) #This returns the id of False, but interpreter printed True last line
>>> (1==1)==True #True doesn't equal True
False, True = False, False