It’s called a “faithless elector” and what happens depends on the law of the state the elector is representing. Some states void the vote without penalty, some void it with some penalty, some allow the vote but with penalty, some allow the vote with no penalty, and some have no law at all (which seems like no difference from allowing with no penalty).
It’s entirely conceivable that enough faithless electors from states that do not void the vote could swing an election, though there’s never been enough to do so before.
I wouldn’t recommend this if you fly very frequently, but you can take some ibuprofen or acetaminophen at the start of the flight / part way though and it should be active around the time you start getting sore.