In the past few weeks, we've been seeing the "Loss of session data" error message in our wiki frequently when users try to save their edited articles (and the save fails). Our best guess is that memcached, which stores session info, is losing session information unexpectedly. This began shortly after upgrading from MediaWiki 1.21 to 1.22 (I don't know if this is related, but the timing is suspicious). When users log out and log back in, the error goes away... for a time. What is the recommended way to debug this kind of problem? What would YOU do if this issue cropped up on your wiki? We've been inserting error_log() statements into MediaWiki core code, dumping the values of $_SERVER, $_COOKIE, and $_SESSION, and haven't found anything helpful yet. Our wiki (version 1.22.5) is served by four webservers behind a load balancer on a private intranet. There's a master and one slave database. memcached has plenty of RAM and therefore should not be dropping data. Thanks for any helpful debugging tips! DanB