PyObject_Size(), PySequence_Size() and PyMapping_Size() can raise an exception. But not always this is checked after using them. This can lead to a crash. For example:

>>> import io
>>> class R(io.IOBase):
...     def readline(self): return None
... 
>>> next(R())
Fatal Python error: a function returned a result with an error set
TypeError: object of type 'NoneType' has no len()                                                                                                                      

The above exception was the direct cause of the following exception:

SystemError: <built-in function next> returned a result with an error set

Current thread 0xb749c700 (most recent call first):
  File "<stdin>", line 1 in <module>