It looks like one of the javascripts in Confluence 2.8 is generating a URI to an image that doesn't exist. The URI follows this pattern:

/display/<SPACE_KEY>/undefined/images/border/spacer.gif

We see a lot of HTTP 404 like this in our logs from these URIs. This is a concern because 404s are slowing down the browser as well as the server (due to CONF-9050).