The NC election is the closest example we have in recent years to an election where fraud (would have) overturned the legitimate results of the election. I say closest because it's not at all clear that the results were enough to overturned what would have been the legitimate result: the stated margin of victory is about 900 votes, and about that many ballots are thought to have been affected.

I will freely admit that mail-in votes are far more likely--about 10×--to be involved in fraud than in-person voting. However, I will also point out that both cases of fraud are quite rare, and far less of an issue in practice than problems such as voter intimidation, voter access, and ballot design, which are known to have afflicted even the largest races in the country (the 2000 presidential election being perhaps the most famous race to have its results altered as a result of poor ballot design).