Cases have occurred where false warnings are being printed for "placer macros" which are shapes created by the placer to control the relative placement of various cells:
WARNING: [Constraints 18-838] Failed to create SRL placer macro for cell cs_oaf2_top_io/inst/ila_core_inst/u_ila_cap_ctrl/U_CDONE/I_YESLUT6.U_SRL32_A. Routing contention at pips element DOUTMUX.
This issue will be fixed in Vivado 2013.3.
Meanwhile, the message can be ignored as the shape is actually being handled correctly.