How to test:

• Install Joomla! 3.5.0-beta (staging 2016-01-17).
• Go to Global Configuration.
• Set Session Lifetime: 1 (minute). Just to wait not too long.
• Open e.g. a banner item for editing
• Do nothing until entered session time has expired.
• Click Save (or was it Save & Close?).
• Backend login screen occurs with fatal error.

Fatal error:
Uncaught exception 'RuntimeException' with message
'The session is not started.'
in /libraries/joomla/session/handler/native.php:221

Stack trace:
#0 [internal function]: JSessionHandlerNative->save()
#1 {main} thrown in /libraries/joomla/session/handler/native.php on line 221