This happened back in 2013, but is somehow just making the rounds now.
Marshall Pope, a motivational rapper was on a live TV show called "Talk of Alabama" and attempted to freestyle rap. Unfortunately for Pope, he completely choked. He started, then stopped, then started and stopped a few more times. Luckily for him, they threw him a bone to get out of the moment, because it was so awkward. And then they invited him back on a few weeks later to redeem himself, which seems like it would be even more mortifying than the original screw-up.