I just found a way to duplicate problem every time, yet I don't think this is what always does it. If you start the signature on the X within the control, the mouse won't track, and when you lift the mouse button, you get a diagonal line from where you released the button down to the lower right corner of the control.
Also, even if you don't leave the form, starting on the X will always fail, but then it draws the line from the mouse button release point to the previous endpoint.
Eric