Halo 1 had error acceleration, but it was primarily confined to the full auto weapons. The pistol and sniper rifle shot true, but they could only be fired at a prescribed rate, so if somebody shoots first, he's automatically ahead of the damage curve for the rest of the exchange. They didn't like that, so with Reach they allow you to fire headshot weapons at faster rates, with the trade-off being error acceleration that was not really a factor for those types of weapons in previous Halo games. You simply weren't allowed to shoot them fast enough for it to matter.