I got the numbers wrong in that description. It wasn't 9999. Anyway, it illustrates the mindset of people who do not seriously expect anything to go wrong. Quoting D. F. Ford, p. 131:
Purely by chance there were some thermocouples . . . in the TMI-2 reactor . . . [normally none, but they were there for a test] . . . If a control-room operator requested temperature data from the computer, he would receive useful information only when the temperature was within the normal six-hundred-degree range. When the temperature got above seven hundred degrees, the computer, instead of reporting it, would simply print out a string of question marks -- "???????." Although the thermocouples could actually measure much higher temperatures, the computer was not programmed to pass these higher temperature readings on to the operators. - Jed
