I guess this could either be fixed up later with a simple check at the end, also linear, just in case it ever happens, or even just a condition whilst placing gates, that they can't be directly opposite the last gate placed but must be some x/y distance away.
Even without any check though, with a 100x100 grid, this is highly unlikely to happen often, as all 50 or so square borders would need to have their gate on the same side of the square, and at exactly the same position.
That's a few probabilities that would all need to intersect, i.e. I believe somewhere on the order of 1 in Math.pow(1/(100*4), 50) unless my probability theory is way off.
