A pinch of not paying attention
A cup of not looking beyond the index page of a folder
An ounce of DOH!
Have an old forum that gets little to no user participation for the last couple of years within a folder of a site. Rarely look at the forum at all, but when you do, just scan the home page of the forum to see if there has been any activity. No? Go do something else.
Watch rankings plummet.
Notice some weirdness in the site: command involving the forum directory.
Proceed to visit the forum, and FINALLY notice that nothing beyond the index page of the forum is working, except some horrible error messages.
Realize that TONS of content has been lost (as far as search engines are concerned), and lots of internal linking back to the home page has disappeared as well.