Sand doesn't fall when placed on a cactus...

When I placed sand (or gravel or anvils) they did not fall when I placed them on the top of a 2-block high (or more than 2) cactus. The cactus block disappeared, and the sand just stayed there. When I removed the bottom of the cactus, the sand just stayed there and it was floating. When I placed a block on top of the sand, the sand still did not fall. The sand only fell when I placed a block below the sand, and then removed it. I have not tested this with other blocks, only the cactus.