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 devices until EDK 12.1.
For more information, see (Xilinx Answer 34533).
This issue is scheduled to be fixed to XPS_CAN_v3_00_a in EDK 12.1. Note that there will not be a version change in EDK 12.1, see the IP changelog to verify changes. Due to ISE design tools dependencies, an 11.5 patch will not be available before 12.1.