My favourite Python misfeature is that `x += y` sometimes does the same thing as `x = x + y` and sometimes doesn't. This leads to minor WTFs like this:

  >>> a = b = (1, 2)        >>> a = b = (1, 2)
  >>> a = a + (3, 4)        >>> a += (3, 4)
  >>> b                     >>> b
  (1, 2)                    (1, 2)

  >>> a = b = [1, 2]        >>> a = b = [1, 2]
  >>> a = a + [3, 4]        >>> a += [3, 4]
  >>> b                     >>> b
  [1, 2]                    [1, 2, 3, 4]

And major ones like this:

  >>> a = ([1, 2], ['one', 'two'])
  >>> a[0] += [3, 4]
  TypeError: 'tuple' object does not support item assignment
  >>> a
  ([1, 2, 3, 4], ['one', 'two'])

The operation raises an exception even though it succeeds!