Make sure you install the snap-rings that locate the gears on the shaft correctly; there is a rounded edge on one side of the ring, and a sharp edge on the other. The rounded side should go against the gear. If they are reversed, the rings can be forced out of their grooves, and allow the gears to slide freely on the shaft, engaging a gear even with the shifter in neutral. It happened to me: the bike moved forward, folding the side-stand and falling over on me, when cranking the motor over with the electric starter during a compression test.