Overused: just about any fashionable 'hard' technology. Remember the stirrup? - some nomad comes to the Bosphorans and shows them stirrups and next thing you know, the entire continent speaks Greek with a funny accent. Same thing as Greek Fire, the horse collar, steam power (did you know there were primitive steam engines in use in the Middle Ages? Yes, OTL. They did pretty much exactly nothing useful). You name it.
Underused: soft technology. If the Romans had had stirrups or gunpowder, they'd probably have gone much the same way they did OTL. But if they had had giro banking or viable social statistics, things could have gone *very* differently indeed.