"It is very important that you use a No. 2 pencil. It is very important that you fill in the entire circle darkly and completely. If you change your response, erase as completely as possible. Incomplete marks or erasures may affect your score."

And here I was, thinking I was done with scan-trons when I finished high school.

But seriously, couldn't you modify a scan-tron's firmware to falsely report results? It's much harder to tamper with every person in an election across wide geographical areas.