The Google Validation Cycle
Google Search Console (GSC) now offers the small webmaster a means to control and monitor how Google spider and index their websites.
In cases where Google determine that there is an error on one of your pages that prevents it being indexed properly GSC gives you the ability to re-submit the page for indexing once you have fixed that error. The problem is that the error is not always obvious. In my case I recieved an error for my Village Hall website as I had not submitted a robots.txt file. This file should specify where the sitemap is located. Adding the robots file and re-submitting seemed to "fix" the error.
When you re-submit a page for re-validation you will get a series to emails from Google telling you the progress and when the error has been validated as "fixed". You will also get an email when an error is detected on one of your sites.
Not maintaining sitemap
In some cases I have added pages and not added them to the sitmap and there have been other instances where I have mispelled the page name and URL in the sitemap. There also have been Canononical declarations on pages that were either wrongly formed or they referred to a page that was not in the sitemap.
The result of this is that Google is likely to throw an error as it cannot find the file/page that you have referenced. These errors can appear in different forms.
GSC errors are different to those errors you may have in your HTML and CSS.