Sometimes things go wrong. We install a module that has a bug, or tweak something in views that breaks the site. How do we diagnose and fix these issues?
Check the log
Watchdog is the logger that tracks what happens in Drupal. It has notices as well as errors. You can use the log to find out what happened before an error occurred.
Reproduce the error
In order for a developer to fix a problem, they need details on how to trigger it. They need to see it for themselves, and be able to check the code while running it.
Uninstalling modules
If you remove a module from git and push to a server that has it installed, it can start throwing errors or crash.
Make sure to uninstall modules on target sites before removing them.
If a module is causing problems and needs to be ripped out, you may need to remove the record from the key/value table so Drupal stops looking for the code.