dupes apparently caught in the tile creation. A dupe is in the a trench doing whatever, and another dupe comes alongs and complete a tile over it. Then another dupe removes the trench. This results in the first dupe hanging by his head with his feet dangling in mid-air. Reload resolves this as I guess the objection collision moves the dupe to a clean spot, but while its happening, it pretty disturbing (and a little amusing) to see these little fellows caught by the head in a tile and its little feet moving hopelessly.
have a dupe work in a trench one block lower then the floor. have another dupe build a tile over the dupe. This should trap the dupe in the tile.