Lorenz's 1961 Numerical Weather Simulation

While rerunning a twelve-variable numerical weather model on an early computer at MIT, Edward Lorenz restarted a simulation partway through using a printed intermediate value rounded to three decimal places instead of the six the computer had stored internally, expecting the tiny difference to wash out. Instead the two runs diverged completely within about two months of simulated weather. The result, published in 1963, demonstrated sensitive dependence on initial conditions in a real weather model for the first time, and became the founding observation of what is now called deterministic chaos.