def a(b=[]):
      b.append(1)
      print b

Now if you do,

  a()
  a()

you might expect to see

  [1]
  [1]

but you will actually see

  [1]
  [1, 1]

The list is not created at function call time, but at function definition time.

This has bitten me more times than I'd care to admit.