I did a clean install of 0.27.176.38a, and made a new world. I embarked on an island and extended the embark area all the way to the deepest part of the ocean possible (~50 levels down, wow). When I use 'k' to view the ocean floor, some of the types very rapidly cycle between 'wet' material types, so fast I can hardly make out what they are saying though it seems there are alot of different gems and such in there. It only seems to be the very very deep parts of the ocean doing this, the land and shallow ocean parts don't have any of these messed up spots that I can find.
I've heard of this bug before, but it's always been someone who tried to overwrite the old version or who was using mods - mine is a clean, unmodded, no utilities used at all install. The only thing I've done is change some init options, and I am using a different tileset.
I regenerated the world in a new folder, new clean install with zero changes to anything (not even the init) and the bug reproduced - there are bugged squares at the bottom of the ocean in the same places.
If you want to try and reproduce it on your own, the seed is 2303250742, the location is here:
Otherwise I can send you a save if you want, just let me know which files you need and where to mail it to. In either case, the bugged squares are easiest to find along the left edge of the map in the deepest square of water.
If anyone else has a moment to try it reproducing it to see if it's just my computer, that would be nice. It's a 0 reject and pretty quick generating world so it shouldn't take too long.