An interesting case just came up as election incident number 024055 in Tulsa, OK in the EIRS. The data merely says, “Smear on ballot next to Republican name, makes all democrat votes register as overvotes, optical scan.”
While futher corroboration is necessary, this points out that the right kind of smudge can mean that seemingly blank ballots are not, in fact, blank. As Doug Jones points out in his wonderful tutorial on optical scan technology, stray marks can affect the resolution of optical scanning. This case is particularly interesting as it biases a particular constituency; that is, the smudge doesn’t affect ballots where the voter chooses a Republican candidate. Further, Democratic ballots will register as having two votes in a race where only one choice is allowed which will invalidate the ballot for that race.