The following error occurs during implementation for 7 series GTP designs in ISE 14.1 and Vivado 2012.1:
"ERROR:Place:1390 - Unroutable Placement! A GTPE_COMMON / GT clock component pair have been found that are not placed at a routable site pair."
This error occurs when a reference clock is shared between the East and West Quads.
To work around this issue, use the reference clock of the same quad. This is expected to be fixed in ISE Design Suite 14.2 and Vivado Design Suite 2012.2.