TYPO3 Core

Set TYPO3 system in maintenance mode by adding the following line in typo3conf/localconf.php
$TYPO3_CONF_VARS['BE']['adminOnly'] = '1';

Log into BE as an admin and go to: ADMINS TOOLS -> User Admin
Switch to a non-admin BE user ("switch-back mode")

Message appears: "Login-error, TYPO3 is in maintenance mode at the moment. Only administrators are allowed access." (see attached screenshot-01). There are no links implemented on this page, so only thing you can do is going "back" in your browser history.
TYPO3 BE re-appears for a few seconds, then the message "Your TYPO3 login is about to expire. Please confirm that you want to stay logged in." pop ups (see attached screenshot-02).

If you click "Stay logged in", you get the "maintenance" message again (see attached screenshot-03) and the "expire" popup re-appears immediately.

If you click "No, log out", you get the "maintenance" message again (loop: only possible next step is: go "back" in browser history).

If you wait until progress bar in "expire" popup is at 100%, TYPO3 asked you to enter your password (for the admin user). Entering the password brings you back to the BE (that's correct) but still shows the "maintenance" message.

At the point described above, where you would go "back" in your browser history, user may be confused and reload the page or enter the URL of TYPO3's BE login again (http://<domain>/typo3). Instead of the login form, the "maintenance" message appears again. So, user do not have a chance to login any more. Only way out is to delete the cookie in the browser.

Firefox version 3.6.13pre used for this test:
Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.2.13pre) Gecko/20101111 Ubuntu/10.04 (lucid) Namoroka/3.6.13pre
(issue imported from #M16411)