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.

The diminishing return approach is this:

     #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...