Towards the end of episode 17 (of the second season of SAO), after extricating Excaliber from the dais in the lowest level of Thrymheim, Kirito makes a comment about the sword "still being too heavy" for him. He then throws Excaliber away before jumping onto the flying tentacle-elephant.
Why did he do that? He doesn't seem averse to accepting the sword from Sinon a few minutes later (after she grabs the sword out of the air with one of her arrows), so it didn't seem like a "I am unworthy to wield this sword" type of thing.