This investigative report purports to have found at least 100 hundred illegal voters - some of whom have voted in recent elections. The report did not reveal in which elections they voted. However, some of these people claim they did not know how they became registered votes.
Part of this problem could be solved by regularly checking and cleaning up the voting rolls. If you remember the story about dead voters in S. Carolina, you’ll remember that this kind of human error is how deceased voters remained on the rolls and some live voters were presumed deceased. It’s not a hard thing to do, but it does require some extra work on the part of election administrators.
Aside from the aspect of human error which can be easily corrected - and the report seems to suggest that these offices are making an effort to do so - how some of these people became registered without their knowledge should probably be looked into as well.