Steps to reproduce:
1. Open the ISE project
2. Click on "User Constraints -> Create Timing constraints"
3. Click on "Advanced -> Slow/Fast Exceptions"
4. Click on constraint and select Delete -> Fatal Error
"FATAL_ERROR:GuiUtilities:Gq_Application.c:579:1.20 - This application has
discovered an exceptional condition from which it cannot recover. Process
will terminate. For technical support on this issue, please open a WebCase
with this project attached at http://www.xilinx.com/support."
This problem has been fixed in the latest 10.1 Service Pack available at:
The first service pack containing the fix is 10.1 Service Pack 3.