Note that you can also get a similar looking error message if the user you are authenticated as does not have permissions to perform the requested operation:
My example below, is an attempt to create a pool using a user whose role is "certificate manager"
[WARNING][60][11 Jul 2014 09:44:51 UTC][RestRequestReceiver][fail] Error processing request for URI:http://localhost:8100/mgmt/tm/ltm/pool
{"code":400,"message":"\"pool\" unexpected argument","errorStack":[]}
[WARNING][62][11 Jul 2014 09:45:37 UTC][RestRequestReceiver][fail] Error processing request for URI:http://localhost:8100/mgmt/tm/ltm/pool
{"code":400,"message":"\"pool\" unexpected argument","errorStack":[]}