Can anyone think of a reason why all the examples are keywords? I would think if you were trying to cause some trouble, redefining standard functions would cause all kinds of chaos, e.g.

  #define memmove memcpy
  #define strncmp memcmp
  #define rand fork
  #define free(x)
  #define strerror(n) strerror((n)+1)
  #define memset(addr, byte, len) memset(addr, ~(byte), len)

The #define else one is scarier to me. #define struct union will almost certainly crash immediately on runtime, as one member's a pointer and it gets overwritten by an int or a pointer to a different type. #define else just always runs all else clauses - the code is likely still valid, it just does something very different from what the author intended.

Both of them would be a real pain to debug.

Both would be easy to notice in a debugger. Finding the cause, as you imply, would be tricky unless you remember to check the preprocessed output.

It's a great memory saver.