I see the below error when running Implementation, even though I did not specify these LUTNM constraints in my XDC:
In this example, the synthesis tool generated incorrect LUTNM constraints.
Ideally the implementation tool should have issued a warning message regarding incorrect constraints, and proceeded with Implementation, as the resulting design would still be functional.
This issue has been fixed in Vivado 2015.1 so that the placer issues a warning message instead of an error and proceeds with Implementation.
As a work-around in Vivado 2014.4, apply different HLUTNM property values for the problematic LUT instances as below.