This bug occurs in both Survival and Creative mode and has been around as long as sand and gravel have been affected by gravity.
If placing sand or gravel blocks in a certain pattern they will overlap when falling and turn into only 1 block.
1. Go to a large, flat, vertical wall
2. Place a block of sand or gravel somewhere upwards on the wall and quickly place another 1 of that block 2 blocks below it.
Their speeds will equal out and when they land they will occupy 1 block and the other will have disappeared.
Edit:This will not work with sand and gravel or gravel and sand. They will bump into each other and not overlap, although sand-sand and gravel-gravel combinations do overlap.