There is not a lot of information around about how broken links affect your web site's performance in the search engines. Search engine specialists routinely check their customer sites for broken links as part of a comprehensive service, but there isn't any concrete data on it.

There's plenty of anecdotal evidence to say that they do have a negative impact, and it's certainly the case that getting listed in reviewed directories such as Open Directory is harder (or impossible) if you have broken links on your site.

PageRank-wise a broken internal link is always going to have a negative impact - you are wasting the "vote" that you have to pass link juice on to another of your pages. After all, a mis-typed link is a connection that you were wanting to make. Additionally it is worse for the user trying to follow it than it is for the search engines.

Personally, I don't think the odd one is going to make any difference to your site rankings, but in the interests of your visitors and the professionalism that you want to promote with your website, then it is something that should be fixed, and it is always something we will highlight in a SEO review.