On some platforms (Darwin, among others), the syscall interface is defined not to be stable. Syscalls must go through Libsystem stubs -- which is written in C.

On some platforms (Darwin, among others), the syscall interface is defined not to be stable. Syscalls must go through Libsystem stubs -- which is written in C. Go made the mistake of assuming the Darwin syscall interface was stable, and an OS update broke binaries.