That is at least in the realm of possibility. They would have to be fraudulently registered (e.g. illegal immigrants). But there are people looking for that as well. There are also provisional ballots from people who are not registered but presumably those have to the checked by both parties before they can be tallied
The most important factor, as with any crime, is cover.
And in those counties in northern VA, they have lots of cover. Once you have cover, (and you’ve done away with conscience) it’s all up to the imagination.