I’m kind of surprised. Normally, these rigged machines wait until the final confirmation step to change the votes, when it would be less likely for the voter to notice.
Think of it this way. You will be committing major voter fraud by tampering with hidden code that changes the vote internally, rather than visibly.
How would you draw attention away from that fraud?
How about making some really, really obvious glitches to generate discussion about faulty machines, glitches that even the dumbest voter couldn’t miss?