There have been server issues on the Archive recently, well, more than usual. If a story not found page pops up, just keep reloading until the expected story appears.
The Svengali Search Utility (link on the left) is programed to deal with the issues.
They apparently have multiple servers, which do not always sync up. This is a common issue on the weekends after they put up new posts. However, something is broken on their end, and this issue is lingering beyond weekends.
Update: The major server issues appear to have subsided. That means we are back to normal operation with their servers only out of sync during their weekend updates. 🙂
Update: Click to See This Post!