We already had one in 2000. This one is likely to be worse since so many states have electronic voting with no paper backup. This makes recounts impossible and any close state will be Florida 2000 squared. There will be several close states.
Well, in the election of 2000, the candidates chosen at the conventions both survived through inauguration. Obviously, only of the slates was elected.
What I was referring to was a scenario where the voters vote-in one candidate, but a different person is elected by the electors and sworn in. That would be unsettling, to never have voted for the person holding the office. It's happened before, when Agnew, then Nixon resigned. It could happen in an even more "dramatic" fashion, before or after the election instead of half way through a term of office, all perfectly legal.