I almost did that once. I was in a FHU RV park for a couple of weeks and the gray filled up faster than the black. So I attached the slinky and opened up the gray tank. When it was empty, I put the cap back on but forgot to close the valve. Before leaving, I emptied the black, closed the valve and put the cap on. Then took the gray cap off to hook up the slinky. Instant lap full of gray water. Amazing how much water can come out of that thing in the two seconds it took me to fix my mistake. Oh, well. It could have been the black tank.