Constraints Editor reports the following Dr. Watson error message when I attempt to select nets to apply the "TPTHRU" attribute:
"Exception Access Violation (0xc0000005) Address 0x0077d564"
Constraints Editor will generally crash if more than 2000 nets are added to a TPTHRU group.
This was fixed in the 4.1i software.
Alternately, you may work around this problem by manually specifying the constraints in the UCF file instead of using Constraints Editor.