1. Can't Save a Case.
Ideally, you should try and fix the problem with the field. There will be a red cross next to the field in error and you can hover the mouse cursor over the red cross to get a description of the issue.
If you can't fix the error for any reason, you can close the offending tab using the x as shown below and try saving again. The information in the tab won't be checked and saved, but you will be able to save any changed information in the other tabs.