Many undervotes were potentially caused by either voter error or errors with the punch card paper ballots resulting in hanging, dimpled, or pregnant chad.
From the en.wikipedia.org
Pregnant or dimpled chad are attached to the ballot at all four corners, but bear an indentation indicating the voter may have intended to mark the ballot.
From the en.wikipedia.org
Republicans have mocked scenes of officials holding up punch cards to see how much light showed through and thus ascertain whether the chad is hanging or merely pregnant.