I have shot a lot of old and questionable ammo and had misfires/hangfires. But I expected that, and was not concerned.
But with good ammo, I expect NO misfires. Theoretically, I might live with one in a million but the one in a hundred mentioned above is totally unacceptable. Something would have to be wrong with the gun or I would never buy that ammo again.
I also would not accept misfires from a new gun. Again, with antique guns, I can accept problems, even accept ones I can't fix. But with a gun I might need for defense, I believe in zero tolerance of misfires.