Quite a number of rooms have been doing that lately. If you do some certain attacks, the tiles seem to change too.
These aren't too bad. I am more bothered by the black squares that you sometimes find behind destructible walls.
The tileset is fine and that case is drawn correctly; you can view the raw pngs for all the tilesets in the tilesets directory. I am exacting about this sort of thing because it bugs the heck out of me too. Please direct your blame toward Nicholas!!
My, my bad! My apologies. Yes, it would be damn illogical for you not to draw select corners of a given tileset. Unless the invisible geometers had a hand in it. Could any of you folks change the thread title to "Nicholas, when I see this, I'm a very sad panda." ? I sure can't.