Common Mistake #5: do not have a backup career plan
Wanting to start a quant finance career is a noble goal, and a goal worth working toward if that’s what attracts you. However, you should always have a backup career plan just in case no satisfying quant job comes your way. In finance, smart investors and traders always have a hedging program to insure against serious losses (whether the hedge works is another question!). When you look for a quant job, regardless of how sure you are of your qualification, a "hedge" is also needed.
Chapter 1 (page 25) of my book gives a couple of examples of such backup career plans.