Yeah, this is a weird bug, but not difficult to get around. It happens when you try to place a decal exactly on one of the diagonals that run from corner to corner across the centre of the map. It can be fixed by switching off "snap to grid" and moving the decal even a single pixel in any direction. Just so long as the X and Z coordinates of the decal aren't exactly the same afterwards, you're ok.
This is also why I find it's a good idea to leave the very centre of the map clear of decals and objects until the very end, so that you can easily see when this has happened.