When the DC is higher than the character can even manage, and if he is really lucky, he can pull off a Critical Success. If he rolls a six on his skill check and still fails, he can roll again using the same bonuses, but with a +2 luck bonus. If that is still not enough for him to succeed, then the DC is out of his ability. Skill checks made on dice larger than a d6 (such as with a focus skill) cannot have an improbable success ƒ they are already improbable as it is.