I ran into this same bug:
I'm building a tall spire. After it was done, I decided to revamp the top 6 floors by expanding them all out some.
The entire tower is constructed from rhyolite blocks, a series of circular floors. At the 18th floor, I destroyed the existing walls and expanded the radius of the circle. I did the same for the 19th floor.
A couple of the walls I destroyed on the 19th floor produced white sand.
On the 20th floor, several walls left behind white sand when they were destroyed. The others left behind a white floor with no material in the description. It's just called " floor". I realized that on those spots, the walls had been built on top of the walls from the lower floor. When I removed the walls from the 19th floor, the walls on the 20th floor no longer had anything under them (though they are supported by the floors of the 20th level, so there's no cave in).
It seems that when a wall is built on top of another wall and that lower wall is removed, the game gets confused about what type of floor should be left behind if/when the top wall is deconstructed. In this case, I'm getting white sand and no-material floors. In reality, there should be open space.
Toady, if you need a copy of the save game, let me know.
Update: the phantom floors are considered constructions. If deconstructed, they leave behind a "white sand" tile, not open space.
[ February 11, 2008: Message edited by: Quintin Stone ]