I have NewRetirement.com set up to send me an email any time the site throws an error (including 404's). Its a great way to ensure that users receive a quality experience with the site, but its also quite informative. For example, you learn quite a bit about search engines by watching the behavior of their spiders. Yahoo! has been notoriosly slow about getting changes propagated through their system. We've moved some content around almost a year ago and set up 301 (permanent) redirects. While the Yahoo! slurp spider comes over frequently, on occasion it still requests old URL's.
What prompted...