The call to PyArg_ParseTuple in _bufferproxy_write uses the wrong formatting string to fill &offset. It uses "i" which is for int, rather than "n" which is for Py_ssize_t. http://docs.python.org/c-api/arg.html
sizeof(Py_ssize_t) on x86_64 is 8, whereas sizeof(int) is 4. If PyArg_ParseTuple is given an "i" it will fill the least-significant (on little-endian) four bytes of the variable, leaving the most-significant untouched. This bug probably wasn't visible on 32-bit x86, where they are both 4 bytes wide.
The end result of this is that _bufferproxy_write ends up throwing a Python IndexError exception sporadically (whenever the uninitialized bytes are not zero) when the arguments passed are otherwise valid.
To reproduce on an x86_64 machine, call it repeatedly until some non-zero garbage value happens to occupy the more significant bytes of offset when _bufferproxy_write is called.
s = some_surface_or_sound
data = some_string_that_should_fit_in_the_buffer
encountered = False
while not encountered:
try:
s.get_buffer.write(data, 0)
except IndexError:
encountered = True
A fix is attached.
