you probably refreshed during
the saving of files. thats how this usually happens. the data is replaced with blank data. its extremely rare. but it does happen. backups are the tools that admins use. admins of every single software.
email address is simple to answer. if the field is "" (blank) then it uses the address from the admin user accessing the page. code for that is within the top of /admin/gen_display.php
if you're curious, open the script file and you'll see it near the top. its a fail-safe rick added for when you access the page for the very first time. this field is the return address for all forum emails.
$config['SITE_EMAIL'] = (!$config['SITE_EMAIL']) ? $user['USER_REAL_EMAIL'] : $config['SITE_EMAIL'];
what mogan may have done probably was:
close his browser during the submit screen.
OR did a CTRL-F5 or forced F5
OR opened a new tab
OR anything that would clear the <input> get/post browser data
...on the GENERAL DISPLAY submission screen in a the browser tab.
when this happens, nothing from the entry screen is passed through to it. essentially submitting "empty fields" and creating "HTML INCLUDES" files of zero-bytes. every tab on the General Display settings page is submitted and written at once. all of the tabs on that page.config-bak.inc.php
was automatically created prior to any update being done. this is the current backup of forum configuration. it gets replaced with the current settings just before new ones are written. if you make changes, the previous settings will overwrite that file.
you should have a backup plan for your entire site. especially your /includes files.
bonus commentary: my backup plan is to backup the settings and scripts every 24 hours. that file is about 5mb total. after that is done, anything older than 30 days is deleted from the directory. this is all done through the CRON tasks on my server. I also FTP in to my sites on a schedule, and mirror the important data to my local hard drives.