This issue might not be reported in simulation and could cause the core to fail in hardware. In summary, the core should not be used for production in Spartan-6 until EDK 12.1.
For more information, see (Xilinx Answer 34533).
This issue is scheduled to be fixed to XPS_LL_TEMAC_v2_03_a in EDK 12.1. Note there will not be a version change in EDK 12.1, see the xps_ll_temac IP changelog to verify changes. A 11.5 patch will not be available before 12.1 due to ISE dependencies.