My fortress was built near a river, with walls on the surface, protecting a single opening. The western wall was right up close to the river, with just a wee bit of gap between them.
Pretty soon, I was ready to add an outer wall for additional protection. I could either knock out some of the inner wall, such that the western inner wall would become the new outer wall; or I could build a whole new outer wall across the river, with the river flowing in between the inner and outer walls.
Decided to build a wall across the river. I know a wall can be supported in this way, by the rest of the wall on the surface (which was already built), so I just needed a place for the dwarves to stand while putting the rest of the wall up. So I built a bridge (literally b g) on the surface, over the river, next to where the rest of the wall would be put up. Then I ordered the whole wall segment at once.
Now, a sensible creature would extend the existing wall one tile at a time. A dwarf, on the other hand, would pick a part of the wall at random, and attempt to build that part, without concern for the rest of the wall.
Did you remember that a bridge does not support constructions? Neither did I.
The mason came along with a stone, and built a piece of wall right over the river, with no support. The stone then fell down, punched through the floor of the river, into the pasture below (nice deep sand layer). Water followed, and did not stop....