Instead, when going diagonally, the mine-cart rails will turn left, turn right, turn left, turn right, etc. etc. I have not tested it in any v. other than 1.4.7, no screenshot, hopefully you guys can replicate it, and then fix it! Mine-craft also did the bug in survival. I had TMI (too many items mod) installed, but it does it regardless of whether the mod is installed. I mentioned that because that was what I had installed when I discovered the bug. (I just use it to change the time of day, and to switch to creative to fly between my structures) I think this is a low priority bug, switch other bugs first.