When my parents and I watched the film version of Harry Potter and the Chamber of Secrets, I notice that Harry Potter is trying to catch the Golden Snitch with his right arm, but the rogue Bludger hit it hard resulting in knocking off a piece of his protective arm gear. (Ouch, it hurts!) But then, he uses his other arm to catch the Snitch and it was a success, but due to his broken arm, he lets go of his Nimbus 2000 and falls on the sand in the middle of the Quidditch stadium. That's why he wasn't aware of this: you can't hold on with a weak arm.