One of the things that gets me going is going to a website, seeing this nice link, clicking on it, and gack, 404!!!! Not only 404, but it's a bland apache page.
Here's a little addition to the WHMCS community, 100% free, no charge, nada. Scroll down to the bottom of the page to find it (it's under whmcs addons)
This mod will also cover 500 errors, 403's, etc. Nice and simple, really, I promise!!!!