Poorly written preprocessor bits in C code that assume that anything anyone ever runs is either Linux, BSD (including OS X), or Windows. They basically write the following:
#ifdef LINUX
// include Linux headers
#elif defined(BSD)
// include BSD headers
#else
// include Windows headers. What else would it possibly be?
#endif
The "mach.h" one is similar: people make a bad assumption that OS X is the only Mach kernel people use. (Hurd is obviously a counterpoint, just an unpopular one.)
I think it's wrong to call it poorly written because the programmer only accounted their C preprocessor code for ~99.99% of desktop and mobile operating systems. The diminishing return for avoiding this approach is so impalpable it hurts.
Windows, OSX/iOS/BSD, and Linux/Android literally are everything anyone ever uses, within rounding error.
#ifdef LINUX
// include Linux headers
#elif defined(BSD)
// include BSD headers
#elif defined(WINDOWS)
// include Windows headers.
# else
#error "unsupported system"
#endif
If you develop for different platforms then it may actually make sense to do a "if linux / elif bsd / elif windows / else #error unsupported". At least it gives you a proper answer in case you forgot you have some wild crosscompiler in your path. But I understand it's not a popular concern...
