When I am synthesizing a design with XST, the following warning occurs:
"WARNING:Xst:1291 - FF/Latch <ff_inst> is unconnected in block <my_block>."
When this warning occurs, a register or latch in your design has been created, but the output is never connected or the signals or logic it drives have been trimmed. Check the XST log for messages such as the following to find signals that have been trimmed out of the design:
"WARNING:Xst:646 - Signal <my_sig> is assigned but never used."