We recently looked at a support call where a large number of 500 errors were being reported - that's lots of red in Integrity or Scrutiny - for a site that appeared fine in the browser.
It was while investigating the request being sent by the browser, we noticed that although the page appeared as expected in the browser, the server was in fact returning a 500 code there too: (this is from Safari's web console)
It's a little odd that the requested page follows hot on the heels of this 500 response code. I don't know the reason for all of this, but if my user finds out and passes it on, I'll update this post.
The moral of the story is... don't take it for granted that if a page appears as expected in the browser, that nothing's wrong. (NB Google will also receive a 500 code when requesting this page.) Another good reason for using a scanning tool.