10.1. Broken Links
Broken links are links that do not work, either because they have been miscoded or because the pages they point to don't exist (perhaps it has been moved).
It's quite important to a search engine that none of the links on your site are broken. It shouldn't be that big a problem to go through your site and check to make sure each link works manually. Doing this will also give you a chance to review your site systematically, and understand the navigation flow from the viewpoint of a bot.
Even though you've checked your links manually, you should also use an automated link-checking tool. Quite a few are available. A good choice is the simple (and free) link checker provided by the World Wide Web Consortium (W3C) at http://validator.w3.org/checklink. All you need to do is enter the domain you want checked and watch the results as the links in your site are crawled.