A few years ago I was involved in creating a website that I figured would be a waste of time from the beginning. However, certain articles on the website had some spelling errors that slipped through the QA.
Rather than fix the errors, I let them slide to see what would happen with the site, traffic, and rankings.
It turns out the site is bringing in over 1000 visitors a day and doing very well in regards to the adsense.
The site has likely paid for itself by now, and hasn't been touched since the launch. It just sits there and generates revenue.
I haven't paid much attention to the people out there making a killing on adsense. To me, this always seemed so small potatoes, it wasn't worth it. I'm starting to think otherwise.
Maybe it would be worthwhile to put up 20 or 30 of these types of sites, and hope that some day I could retire and watch the adsense revenue come in.