"The VT-100 console used to enter Therac-25 prescriptions allowed cursor movement via cursor up and down keys. If the user selected X-ray mode, the machine woul

"The VT-100 console used to enter Therac-25 prescriptions allowed cursor movement via cursor up and down keys. If the user selected X-ray mode, the machine would begin setting up the machine for high-powered X-rays. This process took about 8 seconds. If the user switched to Electron mode within those 8 seconds, the turntable would not switch over to the correct position, leaving the turntable in an unknown state. ... It’s important to note that all the testing to this date had been performed slowly and carefully, as one would expect. Due to the nature of this bug, that sort of testing would never have identified the culprit. It took someone who was familiar with the machine – who worked with the data entry system every day, before the error was found."