People do make mistakes. It happens. Nobody is perfect (and people might also get nervous at such special occassions that ar enot their usual playground. A baseball stadium filled with people who don't necessarily like you as a musician can intimidate even the best of singers.)
X Japan didn't really perform Art of Life live that many times though, in its full length. And I think we will never see that song performed again in full length, because it would be too much for Yoshiki's health to drum and play the piano for a full 30 minutes without a single break. The other members get to have breaks in that song, Toshi also doesn't have to sing the full 30 minutes, because there are guitar solo breaks, piano break and also the verses read by the female voice, but Yoshiki performs all through the 30 minutes.