Back in the days of punch cards, I was working as an intern. There were several large trays of cards that were sitting untouched for a long time. I was told that two sets of unrelated data had accidentally been collated together into more-less-random order and no-one had come up with a way to separate them other than manually. As there were perhaps 20,000 cards the other operators were doing their best to ignore them.
While the others were at lunch, I looked them over. The most obvious difference was color, one set buff, and the other set blue. Then I noticed that one set had a corner-cut on the left and the other set on the right. Poking into the card sorter, I found I could loosen one of the metal hole sensing brushes and cock it to one side to sense the cut corner. In just a few minutes I had the two sets cleanly separated and back into separate trays.
When the others returned, I pointed to the result. They asked me how I had done this. I told them I had set up the sorter to sort on color and never did tell them the real story.