The following Fatal Error is known to occur in Place when cores are used in a design that were generated for a different device:
"FATAL_ERROR:Place:Pl_Design.c:2988:126.96.36.199 - Unable to place this RPM. Due to its structure a packed grid cannot be constructed, hence its group cannot be created and placed. Process will terminate. For technical support on this issue, please open a WebCase with this project attached at http://www.xilinx.com/support."