I know one actually. He sucked in HS and was completely lazy. After high school, college was the last thing on his mind but he quickly realized that he had no job prospects. He had a lunch cart, and decided that he didn't want to spend the rest of his life doing that, so he enrolled in community college.
Of course, he didn't learn anything in high school, so he started with Algebra and basic math, science and history for the first year. From the second year, he started taking more advanced courses (though no pre-med requirements yet). He was hoping to just get an associate degree. But like I said, he did not know a damn thing, so he had to work his butt off even in algebra class. But I think the fact that every morning he had to wake up at 4AM and slave over a stove for 12 hours before going to school at night motivated him so he kept at it.
At the end of his two years, he had like a 3.9 GPA, and people convinced him that he could do 'real' college work, so he transferred to a state college. He still didn't know what he wanted to do (he was a history major when he enrolled), but somewhere along the way he decided to do something in health care (he was thinking nurse).
But as he was used to working like 16 hours a day, he didn't think it was a big deal to do the same in his basic science courses and he was generally the 1st in his class through physics and chem (especially as his fellow freshman were too busy gaining 15 pounds and slacking off). He actually thought that he was dumber than all them "college kids", so he just focused so much until one of his professors noticed and told him to try out for medical school.
He thought the dude was joking as it never crossed his mind that he could do it, but the professor told him to take the rest of his requirements (ochem, biochem, some basic calculus) and see what happens. Well long story short, he worked like crazy in everything and graduated college in about 5.5 years (due to him taking stuff like algebra and basic english in community college the first year), with an insane GPA of 3.91 (4.0 BCPM).
Now he's a third year med student at a very good school. Always found him inspiring.
Now remember, this guy is an extreme outlier. I mean if you drew a bell curve, this guy would be beyond 2 standard deviations from the mean. So this type of think doesn't just happen every day, but it can happen.