It's hard to know which "mistakes" to which you're referring (for example there's a different set of mistakes for each year of medical school, basically), but to start:
1) Accepting the "pre-clinical grades don't matter for residency" sentiment
2) Underestimating their ability to do well, which is usually due to effort put in, not intelligence
3) Failure to get involved substantively in at least one extracurricular activity
4) Taking yourself too seriously, alienating classmates & friends, being too competitive
5) Convincing yourself you can compromise your integrity because everyone else seems to be
6) Overconfidence that you want to go into something noncompetitive and accepting noncompetitive numbers as a result.
....plus I think there's a book of "250 common mistakes of medical students" or something, I recommend looking that up it seems like people have recommended that book in the past.