To Boldly Go
On Sunday, July 22, 1962, the Mariner I Venus probe, intended to be the first American spacecraft to visit another planet, was launched. Two radar systems, one onboard, one ground-based, helped guide it. At Cape Canaveral, Florida, a computer processed signals from the radar and sent control signals back to the tracking system, which sent signals to the rocket.Unfortunately, the timing for the two radar systems was off by a tiny fraction of a second. To synchronize the systems, the controller program added that tiny time difference to the onboard system's information. However the controlling program itself was incorrect because the engineer creating the original, handwritten guidance equations that were the basis for the controller program had left out a single symbol.
During launch, the onboard hardware failed. Which would have been okay, except that, because of the guidance-equation error, the computer was processing the tracking information incorrectly. So, thinking that the rocket was fluctuating erratically, the computer compensated by sending it correction signals. The rocket, which had in fact been ascending smoothly, began to display genuinely erratic behavior, which led the range safety officer to destroy it. The spacecraft perched on the missile's nose, the eighteen-million-dollar Mariner I---still capable of reaching Venus---plunged into the Atlantic. All because one symbol was missed.
Journal of the Plague Years