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A lace card is a punched card with all holes punched (also called a whoopee card, ventilator card, flyswatter card, or IBM doily). They were mainly used as practical jokes to cause unwanted disruption in card readers. Card readers tended to jam when a lace card was inserted, as the resulting card had too little structural strength to avoid buckling inside the mechanism. Card punches could also jam trying to produce cards with all holes punched, owing to power-supply problems. When a lace card was fed through the reader, a card knife or card saw (a flat tool used with punched card readers and card punches) was needed to clear the jam.
Another, possibly more significant use of lace cards was to test (or sabotage) computer programs with little or no data validation.
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