This question already has an answer here:
Though there are significant real-world scenarios to consider, let's keep this question hypothetical.
The situation: a US presidential election is held, votes are tallied, a winner is announced, the losing candidate concedes to the winning candidate, the winner is sworn in and they move into the White House. For some reason it is then found that the tally is incorrect. Key counties in swing states were found to have initially reported incorrect numbers. The differences in tallies are enough to have swung those states to the opposing candidate, and thus the the electoral count would have favored the opposing candidate.
It's understood that the electorates did the real voting and this situation still doesn't change the tally of the electoral vote.
However the group of electorates are sent from a given state is determined by which party held the majority of votes for the state. Therefore, a vote tally error at the state level in this situation results in an entirely wrong group of electorates being sent from that state. Those representatives technically were not the ones chosen to cast the electoral votes for that state.
What's the most extreme situation that could arise from such a scenario? Could an election be invalidated or are we stuck with a person once they move into the White House?