1. Flush Cache: Drupal caches all the content and files to reduce amount of processes and SQL queries for better performance. These cache settings can be set in the Performance page at ( /admin/config/development/performance). These caching are usually turned off in development site and turned on in live production sites. So when you do make changes to the CSS or JS on live site, you need to flush the Drupal cache to see these changes.
Other changes like adding information to the .info file or template file in the theme folder also require cache to be flushed. So in general if any change is not reflected the way it should be flush the cache as a default troubleshoot step.
2. White Screen of Death: One of the common errors which developing (like a custom module) in Drupal is white blank screen nicknamed “white screen of death”. If you encounter it first thing would be to read this tutorial and also enabling error reporting will help in finding the source of the issue.
3. System/Module doesn’t work as it should be: Since Drupal is an open source system the core, themes and modules are being constantly developed with community feedback and contribution. So it’s prone to errors and unexpected results, although most of it is reviewed by the community before making it to the live version. Also it’s these errors you encounter and report back to the community is how the core/themes/modules get a further stable release.
When you encounter these errors the safe thing to do is a Google search with the exact error message you encountered. You may also have to paraphrase the error to land on the right solutions page. These problem/solution threads are usually in the Drupal.org website or in stackoverflow. Also there are many independent blogs which contains solution to problems.
4. Backup Database and files: It is very important that you backup your Drupal site files and Database regularly. If for some reason you site is down due to for example a module update, restoring your previous Database will bring your site back up. This may an extreme situation because you need to always (always I cannot stress this enough) do your module and other updates on a staging server first before doing it live. Even then things may go wrong so take a backup everytime.
5. Slow performance: When making a site live, one of the basic things to do is to enable caching and also compressing the CSS and JS files. These settings can be made in /admin/config/development/performance page of your site. Enabling these will improve the site load times and should not be ignored.