In the ARM documentation this is referred to as “UNPREDICTABLE”. The outcome is not defined. It may work. It may not. It may put garbage data in a register.
I've edited/clarified the claim above. Parent did say "without you having a bug" and entering an UNPREDICTABLE state would be buggy code. Also, "maybe puts garbage data in a register" is not UB, it's a much more reasonable thing in UB (the definition of UNPREDICTABLE prior to ARMv8 does seem to allow for UB, however). I don't believe that such an UNPREDICTABLE state is reachable from user-space/unprivileged code (or else it would violate the chip's security properties) -- but if I'm wrong on that I'd be interested in an example.
There most definitely is.
In the ARM documentation this is referred to as “UNPREDICTABLE”. The outcome is not defined. It may work. It may not. It may put garbage data in a register.