Fooling the grocery store lasers

The conveyor belt at the grocery store checkout has a small hole drilled into the side rail near the clerk where I imagine a light beam, perhaps an invisible laser, crosses just above the belt to detect shopping so as to stop the motor. Yesterday evening I was interested to see the belt advance a little too far and mess the shopping up some. The offending items were bottles of transparent liquid: I wonder if the sensor did not quite see them as sufficiently blocking.