I admit this is with some hindsight, but it was no secret that her lead in the polls was soft, that with undecided voters and third party voters who might(and many didn't) vote third party, that there was potential that things could go wrong. The first goal in any contest is to win. What tends to happen(and we saw it in this election) is when you start thinking about managing to win by alot, you start shoot yourself in the foot. A few more electoral votes past 270 are meaningless. Ensuring you get to 270 is the goal, nothing else matters. Trying for that mandate(which only those who supported her to some degree would even think existed) meant, in part, that she not only did not get a mandate, but that she has no further voice in policy.
There are a number of lessons to be learned from this election. I think that is far and away the biggest one. Winning is everything. Winning with 270 vs winning with 320 makes little significant difference. Just win. I would add other lessons:
2: Running a candidate that is under investigation is a nogo. Guilt does not matter, the harm is done.
3: Learn to better articulate issues, and how to differentiate yourself on those issues. This needs to be done in short sound bites(which is stupid, but it is what it is kinda thing).
4: Negative campaigning is effective, but you also have to build yourself up while tearing the other guy down.
5: Never, not ever, in any circumstance, in any situation, public, private, whatever, say anything negative about voters.
6: No matter how secure you might think a form of communication is, it is not secure. Write and speak as if your words will be broadcast, and expect all staffers to do the same.