Yesterday I received an email from one of my clients asking why the blocks weren't showing up on a 404 page (he went to a url that did not exist). Now I had always known that Drupal doesn't show blocks on the 404 page but I had no idea why and no idea of how to re-enable blocks on the said page.
Recently I was developing a new site for a client that had exceptionally high processor usage for every page load. Now I have other Drupal sites on this server that use a similar module setup but use a lot less processor. My interest was piqued.