This post is a tip for people who work with KML a lot and generate KML from sources other than Google Earth. This could be programmers generating KML through code, people manually creating KML files by hand (a tedious job, but doable), or people getting KML from third party applications.
Google Earth has a setting called ‘KML Error Handling’ which decides what to do when opening a KML file with errors or features that Google Earth does not recognise. It is well worth keeping this set to ‘show prompts for all errors’ to double-check that you are producing good KML. It won’t catch all problems, but it can catch issues that you might otherwise overlook.
If you don’t care about KML errors then it is best to keep the setting on ‘Silently accept all unrecognised data’ as having Google Earth notify you of every error can be annoying, especially if you have an error in something in your ‘Places’, as Google Earth will warn you every time you open Google Earth. In addition, one of the built-in layers in Google Mars contains some KML errors and will pop up a message every time you switch to Google Mars.