Impossible to counter? Really
? I'm pretty sure it was a counter punch over a jab that hurt him against Maidana. Actually, most of the right hands Maidana landed were counter punches.
Maybe you should watch some of Khan's fights properly.
Every punch that Prescott landed to knock Khan out was a counter punch. Over his jab, counter left to Khan's left, counter the jab, right hand and left hook.