Seeing "Warning: include(): Failed opening 'File.php' for inclusion" errors in your logs?Check your cache settings in local.xml. The correct value is "File", otherwise it will still work because of the fallback but look up a non-existing file and flood your logs)

gistfile1.xml

<config>
  <global>
    <cache>
      <backend>File</backend>
      <!-- not "Files", "files", or "file" (Although this mentioned in app/etc/local.xml.additional. It will work because it will fallback to the file backend, but it will flood your error log... -->
    </cache>
  </global>
</config>