An intermittent error in Constraints Editor occurs when the "Group elements by element output name" button on the Advanced tab is used; the following error message will be reported:
"Fatal_Error:utilities:utilarray.c:216:1.3 - sort produced and unsorted list"
This is a known issue that occurs occasionally in 4.2i with Service Pack 1 applied.
If you encounter this problem, you may work around it by writing the constraints by hand. Although this method may sometimes be slower, it will allow you to avoid this error.