Dumb specs, dumb programmers, dumb users
Charles Miller has brought up the 301 problem. Lots of aggregator developers have been burned by the hotel proxy problem. The problem is that as a developer who read the spec, you tend to think that other developers did the same. The interesting thing is what to do with the 301. So you ask the user if they want to accept the redirect. Does the user even understand what that means? Let's see, "Some server is saying that the resource at http://example.com/rss.xml has moved to dumbhotelproxy.com. Do you want to change your subscription?" Considering the amount of effort people are putting into making it easy to subscribe to an RSS feed (because the users don't know what to do with XML, or the orange XML button), I have no idea how to pose that question such that the user might actually do the right thing.
Funny enough, the second case Charles bnings up actually is exactly what Outlook does with the ItemRemove event. You get the event, but not the item that was deleted, because it's already gone.