Not that I understand astrophysics mathematical calculations, but it seems that I don't really need to. I mean, if NASA can be off their calculations by a magnitude of 1000, then what do I need with that kind of science. Let's just leave it to the 13 year old German boys to predict the end of the world.
DailyTech - German Schoolboy Finds Fault in NASA's Apophis Calculations
heads up: this is a pretty old post, it may be outdated.