He's not the Oracle at Delphi; he gives probability estimates, not inescapable prophecy. It's inevitable that some elections will go to the candidate who he predicts has a lower chance of winning. The real question is, how often? If it happens more or less often than he predicts, then his predictions are biased and should update on that fact.

One factor that messes with any model is lack of consistent data. House races aren't publicly polled nearly as often as statewide polls. And furthermore, it's not necessarily that he "got races wrong." When he says Obama wins 80% of the time, he's also saying that Romney wins 20% of the time. 1 out of 5 times, Romney will win and Obama will lose, despite being a favorite to win.

What you just posted is misleading. Silver's model gave the GOP a 2 in 3 chance of winning the House in 2010, predicting a net gain of 45-50 House seats, and the majority - http://fivethirtyeight.blogs.nytimes.com/2010/09/10/g-o-p-ha...

Also, his modeling is MUCH more accurate for presidential races, where there is more polling data, and thus a lower sampling error.