Yes - it's been done for years.

Some of the first golfers (known then as "programmers", or sometimes "hackers") would use invalid opcodes to make their programs faster and take up less space (two things almost synonymous back then, ignoring the algorithm and loop unrolling). I won't go into details about the benefits and widespread use of such undocumented opcodes - you can read more here.

Banning this golfing technique would be akin to banning closures in a language where they were an emergent feature-not-bug of the original implementation, and an unofficial clean-room reimplementation from the language documentation didn't support them.