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.