I think there is a "bleed threshold" which can be adjusted on the BMD machines which senses ink bleed through the paper and rejects it past a certain point.  If you are in a purple-red precinct, you know that Dems are gonna vote early en masse and GOP are gonna vote in person.  Switch out the pens on election day and you will reject a very large portion of the R ballots.  Sneaky mofos.