If a student can make a decent case that he or she simply got out-of-synch with the bubble sheet, then the penalty should be no larger than an effective deterrent for such mistakes.
(I used to work for a professor such that, when a student answered a question wrong, we were to see where he or she got things wrong, deduct some points for that, but then see if what followed made sense had the mistake not been a mistake, and give credit t=if it did.)