I saw this happen once unintentionally. We went right from the start, and all the way down that hall where the boss patrols. The tank had his back to the wall, and was knocked back over the ledge and below the instance. He described being able to walk through walls down there, was with all of the constructs down there, and could see the NPCs above him. The group that knocked him off stayed targeting him but evaded all attacks and could not hit him. He ended up having to hearth to get out.