I thought this was a problem due to stale security and session cookies, but now I'm not so sure. On Mac Chrome I've seen and had reported a problem where when attempting to edit an asset the user is confronted with an invalid security token notice:
Our authentication is using the LDAP bridge, with the apache remote user value being used to determine the current user (it's set via a process which works well locally).
The problem seems to be specific to Chrome on the Mac - using Firefox the problem doesn't occur. I don't think we've seen the problem on Win Chrome.