a very low priority bug;
When rotating some pieces in the editor; they move very gradually to the left. I noticed this with the triangular and pillar end pieces from original Pillar set. But the wooden planks don't seem to do this.
When determining the position of a piece that got rotated around its center, one has to round the piece position, if either the width or the height of the piece is odd. What you encountered is, that a piece moves slightly if these "rounding unprecisions" accumulate.
I don't think there is any good way to solve this issue without adding a lot of code. So I would simply say: Don't rotate too much :P