Was that the one about a kid--I think his name was Tertius--whom Merlin was trying to teach magic (or something) but somehow instead wound up accidentally imbuing with all kinds of twentieth-century scientific and technological knowledge, which was largely useless (at best) for the era he was actually living in? As I recall, the viewpoint character was a more "normal" Arthurian-era kid who becomes friends with Tertius while they're trying to find some way to use Tertius's anachronistic knowledge to deal with some current problem the Knights of the Round Table are attempting to solve.