This happened in London, on Android version of MM: I was dragging my precious interchange onto the map, when another of my fingers accidentally touched the screen in some random spot... causing the interchange to drop at a random spot. Of course, to my dismay!
However, to my surprise, another interchange miraculously appeared in my assets at the same time, and I could drag it to the desired location. So I have 3 interchanges, on the London map. I've never been offered a 3rd interchange in London. I think it's a bug, and I'm now wondering if that's how some people have managed to get the huge scores on the leader boards?