General Description:
When I try to save a UCF that has OFFSET OUT BEFORE and OFFSET IN AFTER constraints, the constraints are changed, causing errors in NGDBuild.
This problem has been fixed in the latest 6.1i Service Pack, available at:
http://support.xilinx.com/xlnx/xil_sw_updates_home.jsp
The first service pack containing the fix is 6.1i Service Pack 2.