I just confirmed this with Skyld/my brother (both using the new client, though my brother isn't using the 64-bit one).
When shooting "over" a wall on a modern server, it means you're getting as close to a wall as you can, and then shooting...
which works fine. However, on the new Mac client, you're able to shoot over 3-tile walls with the same guns as other players who are not able to shoot over it, with the same x/y as far as I can tell. On the screen of the people shooting the gun over the 3 tiles, the bullets do not appear to pass. However, for everyone else, the bullets do pass, and they can be hit by them.
Definitely something that should be fixed, otherwise a ton of origins will have to be changed on various servers.