This sounds like sightreading vs memorizing songs (musically or DDR-ish). There are some games where I just can't get past certain parts, because it's too fast to sightread, and I can't memorize the sequence (the Battlef*ing toads speeder level is a good example, especially with two people), but there are others that are hard to finish if I miss something, because I've played it so much I've lost the ability to sight read it (Contra and Life Force, mostly)