The Supreme Court has upheld the rights of states to punish "faithless electors" in the Electoral College; this is a bad idea in general. It is not necessarily disastrous in itself, but depending on state laws governing how pledging to vote a certain way works, it could end up being so at some point. The actual breakdown of faithless electors historically is something like this, assuming I haven't missed or double-counted any:
63 (in 1872) were due to when Presidential candidate Horace Greeley died after Election Day;
30 (in 1832) were protest votes against Martin Van Buren;
23 (in 1836) were protest votes against the Vice Presidential candidate Richard M. Johnson;
8 (in 1912) were due to when Vice Presidential candidate James Sherman died after Election Day;
7 (in 1828) were protest votes against John Calhoun;
6 (in 1808) were protest votes against James Madison;
5 (in 2016) were protest votes against Hillary Clinton;
4 (in 1896) were due to the fact that two different parties, the Democratic Party and the People's Party, endorsed the same Presidential candidate but different Vice Presidential candidates, and some People's Party electors decided to vote the Democratic slate for the latter office;
3 were protest votes against Richard Nixon;
3 (in 1812) were protest votes against Vice Presidential candidate Jared Ingersoll;
2 (in 1832) were protest abstentions against Henry Clay;
2 (in 2016) were protest votes against Donald Trump;
1 (in 2016) was a protest vote against Vice Presidential candidate Mike Pence;
1 (in 1820) is supposedly because the elector believed that giving the votes unanimously was not usually reasonable;
1 (in 1948) was due to the fact that the Tennessee Democratic Party had a schism;
1 (in 1988) was to draw attention to the fact that electors could vote for someone other than the person who won the popular vote for the state;
1 (in 2000) was a protest for the lack of Congressional representation for the District of Columbia;
the remaining 4 were for reasons unknown.
Of all the 165 faithless electors, seventy-one were only 'faithless' in the sense that they had to decide who to vote for when the candidate for whom people had voted had died; naturally, they switched their votes to the next party choice. As I've noted before, almost all of the others mark elections in which there was widespread discontent with at least one of the candidates, so that the Electoral College is actually capturing a sense of the people that would otherwise be ignored. This registering of discontent at the circumstances of the election itself is not something most election systems do (in general, I think, only elections with 'none of the above' options do better at it), but it is quite valuable for taking the measure of the election. And as far as I am aware, the Electoral College is the only election system that has registered the fact that there are people who don't like the election system itself. But the poorly written state laws requiring Electors to follow the state-certified popular vote (and they seem generally to be poorly written) do not adequately take into account these kinds of situations, or indeed, any situations that the mediocre and not-notably-creative minds who usually make up state legislatures cannot foresee.
