Invalid Form Key, Please Refresh Page at Checkout Magento 1.9.1

Question

i havent changed any cookies settings that some people have mentioned to try. I did install memcache, im not sure if that is related.

local.xml

<config>
    <global>
        <install>
            <date><![CDATA[Fri, 17 May 2013 03:32:24 +0000]]></date>
        </install>
        <crypt>
            <key><![CDATA[RANDOMNUMBER]]></key>
        </crypt>
        <disable_local_modules>false</disable_local_modules>
        <resources>
            <db>
                <table_prefix><![CDATA[]]></table_prefix>
            </db>
            <default_setup>
                <connection>
                    <host><![CDATA[localhost]]></host>
                    <username><![CDATA[USERNAME]]></username>
                    <password><![CDATA[PASSWORD]]></password>
                    <dbname><![CDATA[DATABASE]]></dbname>
                    <initStatements><![CDATA[SET NAMES utf8]]></initStatements>
                    <model><![CDATA[mysql4]]></model>
                    <type><![CDATA[pdo_mysql]]></type>
                    <pdoType><![CDATA[]]></pdoType>
                    <active>1</active>
                </connection>
            </default_setup>
  <!-- START new connection to wordpress database !-->
            <wordpress_setup>
                <connection>
                    <host><![CDATA[localhost]]></host>
                    <username><![CDATA[USERNAME]]></username>
                    <password><![CDATA[PASSWORD]></password>
                    <dbname><![CDATA[DATABASE]]></dbname>
                    <initStatements><![CDATA[SET NAMES utf8]]></initStatements>
                    <model><![CDATA[mysql4]]></model>
                    <type><![CDATA[pdo_mysql]]></type>
                    <pdoType><![CDATA[]]></pdoType>
                    <active>1</active>
                    <use>0</use>
                </connection>
            </wordpress_setup>
            <!-- FINISH new connection to wordpress database !-->
        </resources>
        <session_save><![CDATA[db]]></session_save>
<cache>
               <backend>ewfile</backend>
           <slow_backend>ewfile</slow_backend>
          </cache>
    </global>
    <admin>
        <routers>
            <adminhtml>
                <args>
                    <frontName><![CDATA[ADMIN]]></frontName>
                </args>
            </adminhtml>
        </routers>
    </admin>
</config>

Answer 1

Memcached in theory shouldn't be used on your checkout. It potentially could cache user data and show it to someone else.

It sounds like your form key might be getting cached into memcached by another user and then when you visit/a separate customer visits the checkout they have the cached form key rather than their own.

It might be worth disabling memcached and then retrying the checkout, if it works then there is most likely something wrong in the configuration for memcached.

Some useful information regarding memcached configuration for magento: https://redislabs.com/lp/magento-memcached/