Let's say I've found a level patch of ground to use my Motion Detector on. Just as I start setting it down, a friendly passes in front of me and nullifies the placement. That motion detector fails to plant, but is counted as expended; as in, I now have one less motion detector without having placed one. Also, Infiltrators cannot place motion-detectors anywhere near a corpse. This is particularly bothersome in clustered fights, or anywhere a MAX suit decides to fall over.