So far, I've found about 31 different incidents (some of which involve multiple ballots) since 2000, anywhere in the country. [...] Some of these 31 incidents have been thoroughly investigated (including some prosecutions). But many have not. Based on how other claims have turned out, I'd bet that some of the 31 will end up debunked: a problem with matching people from one big computer list to another, or a data entry error, or confusion between two different people with the same name, or someone signing in on the wrong line of a pollbook.
These conspiracies were the pretext for the voter ID laws that have now been passed in 34 states. And the arguments in many of those states have reached an absurdly high pitch. In Virginia, for example, Republicans are saying that the ID card required in their law has to be current; if you happen to let your driver's license expire, you can't vote, even though the photo on the card clearly demonstrates your identity. The state's Democratic attorney general, Mark Herring, says that's unconstitutional. But neither the Constitution nor plainly visible reality is likely to halt the Republican crusade to keep certain people from participating in democracy.