If it was undefined, then anything could happen (nasal demons), but that would be silly: you wouldn't be able to safely call functions with multiple side-effect

If it was undefined, then anything could happen (nasal demons), but that would be silly: you wouldn't be able to safely call functions with multiple side-effecting arguments at all. Unspecified means that you can't rely on the order of evaluation. "Anything" may not happen; the compiler controls the sequencing.