A story about Magic (Jargon File)

A hacker recounts finding a small two-position switch glued to the cabinet of an MIT AI Lab PDP-10 labeled "magic" and "more magic." The switch had only one visible wire and, on inspection, that wire terminated at a ground pin, so it appeared electrically inert. Assuming it was a joke, the narrator flipped it and the computer immediately crashed; they restored it and recovered the machine. A year later, showing the same switch to David Moon, they again flipped it after confirming its lone grounded connection and the machine again crashed. Richard Greenblatt then cut the switch out with diagonal cutters, the system was restarted, and it ran reliably thereafter.

The team never determined a definitive mechanism for the crashes, but the concrete findings are that an apparently nonfunctional, single-grounded toggle repeatedly produced synchronous failures and that permanent removal eliminated the fault. The leading technical explanation offered is that changing the switch’s position altered local capacitance or electromagnetic conditions enough to upset a marginal circuit during high-speed pulses. The narrator kept the removed switch as a memento, usually set to "more magic."