You found many more buggs, thanks. You were not supposed to be able to push the block while on a different side. You also put a box inside another box of the other side and they counter each other, which means there's no collision, but the box is still visible on one side, when it should just disappear. You couldn't find a solution probably because you didn't know what was a bug and what was supposed to happen, but you were on the right track. If you think about it, you could find a way of getting that corner cube which you took by means of bug.