I can't work out if this could be useful or not!
In AC11/12 use a line or polyline set to rectangle or rotated rectangle mode. Click once to place first corner. Hold down the spacebar to temporarily turn off guideline snapping. Click again to place the other corner. You will notice your 'rectangle' is missing the side above / below the start point - becoming a three sided polyline.
Is this a bug, or could this have some kind of use in the real world?