Opening an existing file in read-only text mode and trying to write to it:

>>> f = open("LICENSE")
>>> f.write("")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
SystemError: null argument to internal routine

Binary files get it right though:

>>> f = open("LICENSE", "rb")
>>> f.write("")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
io.UnsupportedOperation: write