There are few things that will make your site look more unprofessional than users following a broken link and receiving a 404 – File not found message. For a good developer it’s easy to ensure that a site is free from broken links. If you’re a Linux user you can use ht://check, if you prefer Windows you can use Xenu and Mac users can try BLT.
However, not all Web developers are as diligent (or competent) as you are. I’m sure you choose Cool URIs that never change and, in those very few circumstances where you do have to change a URI, you put in a proper server-side redirect. However, what about those other Web developers, the ones that link to you, how many of them check links after they’ve created them?
What do you do if another developer makes a mess of linking to you so that it causes a 404 Page Not Found error on your site?