TL;DR: if PC user compiles source which defines "timezone" in a header, and includes Python.h after the header, then the macro also called "timezone" in pyconfig.h will redefine the users source, yielding unexpected results.

see Cython google group for an actual use case where this occurred:
https://groups.google.com/forum/#!topic/cython-users/pjbhYrpl_rU

trying to compile NREL SPA code which has a header file with a struct with a timezone field fails with the following traceback:
spa.h(88): error C2032: '__timezone': function cannot be member of struct '<anonymous-tag>'

here's the macro in PC/pyconfig.h in master as of 5489bdad
https://github.com/python/cpython/blob/5489bdad5143050b8bb89b648b3c00d951b72d4f/PC/pyconfig.h#L202

/* VS 2015 defines these names with a leading underscore */
#if _MSC_VER >= 1900
#define timezone _timezone
#define daylight _daylight
#define tzname _tzname
#endif