This bug occurs in both Survival and Creative mode and has been around as long as sand and gravel have been affected by gravity.

Description:

If placing sand or gravel blocks in a certain pattern they will overlap when falling and turn into only 1 block.

To reproduce:

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.