“Voters who vote by mail are not able to use ballot checking technologies. They may make a simple mistake. There may be a stray ink mark. The ink may smear when it’s stuck in the envelope.“
A simple smear of ink can be enough to have a vote thrown out altogether.
Lever machines are the best… toggling the switches and throwing the mechanism is much more thrilling than SAT-style dot-filling. My first time was with a lever machine – I get really nostalgic for them here in NYC, although I suppose their error rate makes them mostly a thing of the past.