Richard Feynman told a story that when he was at Los Alamos during WW2, he was put in charge of the nascent computing group using the brand new IBM computers to

Richard Feynman told a story that when he was at Los Alamos during WW2, he was put in charge of the nascent computing group using the brand new IBM computers to run physics calculations. The army recruited a bunch of high school math whiz kids to handle all the punch card loading and running the machines, and Feynman was in charge of the kids. Eventually the pressure from Feynman to keep up the pace of calculation was so great that the kids, keeping track of program progress in their heads, would yell at him anytime he came into the computer room to ask a question because it meant losing track and having to restart the programs which were many, many cards. This meant losing days of precious time if they had to start over or if a program broke. Finally the kids got so stressed debugging (and failing to debug) these long running programs that they devised a way to run multiple programs at alongside each other by color coding the punch cards, and then recording break points in the programs when they failed. This allowed them to keep the machines always running with one kid in charge the different variants of the programs they were running.