Actually it does. BSF (Bit Scan Forward) is the most famous which returns an undefined result if your operand is 0. Many instructions leave the state of bits

Actually it does. BSF (Bit Scan Forward) is the most famous which returns an undefined result if your operand is 0. Many instructions leave the state of bits in the FLAGs register undefined. E.g. AAA will leave SF,ZF, and PF undefined. Then of course there is the “reserved bits” but those are easier to avoid.