This screenshot displays a long term date range with number of pages indexed (grey), crawl errors (in red), an appearances in search (blue). This data is from Bing Webmaster Tools
SEO Report and Analysis Tools
This graph is a printout for Graphic Web Design, Inc. Notice the increase in crawl errors around the end of September 2012. This was when I edited the .htaccess, removing 301 redirects. This is not something that would normally be left unattended, but I enjoy running experiments on my website. You can see a decrease in the number of pages indexed, and appearance in search results.
At the end of November 2012, our website was migrated into Drupal. Since this time, I have monitored 404 errors. When a 404 occurs, and 301 redirect is added.
In 2009, this website was launched in HTML. About one year later, it was migrated to a simple PHP website. After that, we went into Joomla. Finally, we have migrated into Drupal. Still to this day, the server is recieving requests for URLs that existed 3 years ago.