...of wooden computers and iron programmers, I had a lady working for me as a keypuncher. She had been a superb typist -- very fast -- but as a keypuncher she had a very high error rate. She also wasted almost no cardstock, which should have been a clue, I suppose. I examined a few of her "bad" cards and pretty quickly figured out what was happening.

First, for those who might have missed the punched-card era, the cards (technically "Hollerith" cards, AKA "IBM" cards) were roughly 7"x3" pieces of stiff paper, in which individual characters were coded as holes punched in vertical columns using a machine called a "keypunch". Each card column held one character; a typewriter-like keyboard on the keypunch was used to tell the machine what to punch. On some keypunches, the character was also printed along the top of the card using a tiny dot-matrix printer. Google "Punched card" for details.

I noticed that on her bad cards, the bad column had incorrect holes in it, but the printed character looked correct. On close inspection, the printed character had the correct character printed more darkly, over some other fainter dots.

I watched her at work, and, sure enough, when she thought she had made a mistake, she backspaced and overpunched the correct character several times, just as you might do on a typewriter. I pointed out that while a human could read the result OK, the machine could not, because the machine read the holes, not the printed character. She was surprised, having had no clue what the holes were for. I showed her how to make corrrections properly, and her error rate went to zero.