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A lace card is a punch card with all holes punched (also called a whoopee card, ventilator card or IBM doily). Card readers tended to jam when they got to one of these, as the resulting card had too little structural strength to avoid buckling inside the mechanism. Card punches could also jam trying to produce these things owing to power-supply problems. When some practical joker fed a lace card through the reader, you needed to clear the jam with a 'card knife'.
A more modern equivalent is a black fax.
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