You’re lazy. It’s okay to admit, because so is everyone else. You don’t want to pour through tour web logs looking missing pages, but you also don’t want to let requests 404 on your visitors. So you sign up for a free Google Webmaster Tools account.
Google Webmaster Tool does a lot of good basic reporting, but when they added the 404 tracking under Diagnostics->Web Crawl->Not Found, it became a must have.Now you can have the Googlebot report to you what it can’t find. It also tells you how many places are linking to that specific url.
No matter how careful you are, there will be times when stuff fails. Check in on your sites at least once a week to make sure it’s nothing huge.
Tonight I saw that 680 404 errors were being reported all because of a typo in one of my redirects – oops. It only took a minute to fix it, but the errors had been there fro 2 days! Luckily they were not my main site, but a small sub-site.
You can even stick a 404 widget on your iGoogle homepage if you really want to be good.