I actually thought it might happen this election. Say Trump had won 270-268, but one of his electors votes for Paul Ryan. Now there’s no EC majority, so the House has to choose between Clinton, Trump and Ryan. In any normal year, the House would automatically just anoint the majority party’s candidate. This year, it’s certainly not impossible that sane Republicans might have joined Democrats to elect Ryan.
IMO, the idea of the Electoral College overriding the results of the election is about equivalent to the idea of the Queen dismissing Parliament and ruling the UK by fiat - the law theoretically allows it to happen, but that law is a historical relic that only still exists because it never became necessary to undo it, and it would throw the entire system into chaos if it were actually invoked.
It’s extraordinarily unlikely to happen, especially given the number of faithless electors that would be required, but…
It’s the most likely this time than at any point in my fifty-mumble years of life: a candidate who is so polarizing and so outside the norms of political operation, combined with an apparent loss or at best razor-thin popular vote margin seem like ingredients to consider.
It may be possible if a group of electors collude to vote for third person. I don’t think they would but for example if the entire Texican slate decides to vote for Cruz it would not be the most surprising thing I’ve ever seen.
Although there are evidently electors who have delusions of grandeur, they’re not going to block Trump from the presidency (especially not that guy). It would show that he was right all along, the system is rigged, and if blood didn’t flow in the streets at least there would be a substantial demographics that would never support a Republican again.