Following the documentation at https://docs.python.org/3/c-api/buffer.html, if we write a custom object in C with a getbufferproc that simply returns PyBuffer_FillInfo(...), then it works fine up to Python 3.2 but segfaults in Python 3.3 depending on what we do with it.  The segfault is caused by memoryobject.c:last_dim_is_contiguous(), which reads from the "strides" array without checking that it is not NULL.

Attached the simplest example of C extension module I could make.  When executing this:

>>> import xy
>>> m=memoryview(bytearray(b"abcdef"))
>>> m[:5] = xy.gm

...it segfaults in Python 3.3.  (I'm told it works fine in 3.2 and segfaults in 3.4 as well, but didn't confirm it.)