I use this simple, very nice plug-in from Ciaran Gultnieks to hide the black bar at the top of the new WordPress 3.8 front-end, and to hide/show the WordPress toolbar:
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After attempting an automatic update to my self-hosted WordPress install from version 3.6.1 to version 3.7, within the admin panel, the site returned a completely blank page... I was unable to log back in to the admin panel at all, and the public site was down as well. The resulting URL displayed in the window contained:
../wp-admin/upgrade.php?_wp_http_referer=%2Fwp-admin%2F
...and in the the error log (viewable via FTP) was the entry:
Call to undefined function wp_get_current_user() in /wp-includes/user.php on line 215
In the WordPress.org › Support » Installation forum, user photoMaldives had the fix posted here... nice job.