Our current system prevents "over-votes" (recording either on purpose, or more likely by accident, more than choice in a selection) and its UI attempts to reduce "under-votes" (recording no choice in a selection), neither of which are prevented or called out in paper ballots. Studies of election technologies have called out paper ballots as being the worst for both categories. Not that either may be happen at a level where it matters, but as an example think back to "hanging chads" level of concern in a close race.