Vanilla Dwarf Fortress 0.40.09 -- The only exception to this is that GAIT for Dwarves has been modified to provide them greater speed. I doubt this should affect this particular error, however. Save starts on appropriate Z-Level -- There is an area of unsmoothed flooring present to the right of the large room with chairs. Setting the smoothing designation on that area, then placing a any Stockpile over the area should trigger the error.

Smooth Floor designation functions as normal when there is no Stockpile covering the area, and will still function as normal when the Smooth designation is set after a Stockpile has been established. However, when placing a Stockpile over an area that has already been designated for Smoothing, the Smoothing designation disappears. This appears to occur regardless of Stockpile type that is placed over the designated area.

Already-smoothed flooring is not affected in any way; This affects ONLY the actual labor designation.