When in a minecart/boat, no matter where you aim, you will end up breaking the boat/minecart if you try to use a bow (in any game-mode) Also, if the boat/minecart is invulnerable, you cannot use any projectiles (snowballs, enderpearls or arrows) when inside because it hits the boat/minecart. I think this is a bug because this was not the case in 1.7. although you could hit vehicles with projectiles while inside them In 1.7, you could still shoot outside them if you aim high enough which is not the case in 14w08a. another noticeable difference is that when inside a minecart on a rail, in 14w08a, your hand appears to be darker in first person F5 mode even if it is bright where you are which is also not the case in 1.7 (as shown in the pictures below). maybe this is because players in minecarts/boats where lowered in this snapshot, meaning the game thinks that you are in the block below the vehicle, and that you hit the vehicle with your projectiles no matter where you aim.