Lindsey Jacobellis
Elite athletes must remain professional and focused throughout their competition. If they relax for a single moment it could produce a career-defining event. Jacobellis discovered this most heartbreakingly when she threw away her chance of a gold medal in 2006.

She had a phenomenal record in the X Games but failed to win first place in the Winter Olympics. Jacobellis built up a massive lad during her final race but then she decided to showboat. This was a fatal decision because she lost her balance and suffered an ill-timed fall (via USA Today Sports).