On my civievent, people get the error message " could not find valid value for id" on wordpress 4.8.1
I had this problem intermittently for some time when embedding event registration forms using a Wordpress shortcode. It has to do with page caching. CiviCRM parses the shortcode to capture the event ID and adds the id value to the global $_GET object. If the page is loaded from cache (usually when the back/forward button is pressed) the code that parses the shortcode does not run, and the event ID is not added to $_GET.
I was able to fix this issue by making a page cache exception with the caching plugin I'm using.
I think this behavior is essentially a bug and CiviCRM should output a hidden form variable with the event ID so that it still gets passed to the "Thank You" page if the registration form is loaded from cache.