Unable to log into admin control panel
Moving to a server (or local dev environment) that doesn't have Redis installed
Install Redis! Pretty straightforward on Linux. For a guide to installing Redis on Mac, check this resource: http://jasdeep.ca/2012/05/installing-redis-on-mac-os-x/ - Or disable Redis if you must by searching for 'redis' in the app/etc/local.xml file.
Getting the error "Function name must be a string" (etc)
If you are migrating from a site that was running PHP version 5.x, try matching that in the new environment for a quick fix. Running on PHP v7+ will bring new problems - it's best to make sure it's running first before dealing with these types of issues. For more info, see https://www.atwix.com/magento/magento-and-php-7/
Find/change the Magento database credentials
Check app/etc/local.xml and steal/change server address, credentials, etc.