Projectiles (the ones I've tested are snowballs, enderpearls, and bow arrows) won't fire if moving between hotbar slots fast. You can still see the projectile moving in the air after thrown/fired, but it will not cause knockback or damage if hits something, just phases through. Also the object will not be lost from hotbar, may vanish and reappear sometime later or just not be delpleted
Could you attach a video?