A more unpleasant variant of this would be to combine `#define i j` with a language like Fortran 77 where variables are implicitly typed by letter. e.g. `#define i x` would change the implicit type from int to float...

It gets debugged in a flash though.

yep. e.g. gcc 4.8.4 happily complains about redeclaration errors and points out that it is a result of the offending macro expansion.