I've tried to explain this before. It's not invisible walls, and has nothing really to do with the editor. It can be avoided.
It happens when there isn't a wall or enough wall thickness around the level. You move a box or something and a ghost of it appears at the left hand side, you can, if you know where it is, move the invisible whatever and use it in the game, as has been done several times before. If you do move the invisible thing, then the real one moves too and, if it was in water, disappears.
Some ingenious levels were made around this bug ages ago, one by majic1 where you had to keep hold of the invisible box all the way through the game and use it along the way.
As long as you know what causes it you can avoid it happening, or use it to your advantage.
