This could be one of thousands of things. If it works with 2.5 and no
more with 2.6 some possible candidates are (1) a bug in Python, (2) a
difference in memory layout which caused buggy code to work correct in
2.5 nonetheless or (3) a timing difference, with the same effect.
Oh yes, if you're on Windows and using the prebuilt dll it could also
be caused by runtime library differences, I think 2.5 was built with
VS2003 and 2.6 with VS2005.
You'll need to provide a bit more detail if you want more help (OS and
version, how you built Python and your app, is your code in an
extension module or embedding Python, how does it crash, did you get a
stack trace, etc)