On secure.phab front page with persistent column open. Trying to unflag an item. Got a popup with this message:
Unhandled Exception ("AphrontCSRFException")
You are trying to save some data to Phabricator, but the request your browser made included an incorrect token. Reload the page and try again. You may need to clear your cookies. This was an Ajax request. This request had an invalid CSRF token.Closing column and refreshing fixes.
Not sure if this is a known/accepted deficiency given the in-progress state of persistent column, but thought I'd log it here just in case.