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Magic smoke (also factory smoke, blue smoke, or the genie) is a humorous name for the caustic smoke produced by burning out electronic circuits or components, usually by overheating, overamping, or incorrect wiring configurations. The smoke typically smells of burning plastic and other chemicals, and sometimes contains specks of sticky black ash. The color of the smoke depends on which component is overheating, but it is commonly white or grey. Simple overheating eventually results in component failure, but does not release smoke. Real smoke is almost almost always the result of incorrect wiring or a manufacturing failure in the component.
The name is a running in-joke that started among electrical engineers and technicians. It was more recently adopted by computer programmers. It is a widely used term with competitors in the FIRST Robotics Competition. The given device operates until the smoke is released from it, at which point the device ceases to operate. Ergo, the smoke is an essential part of the device's operation, through undetermined ('magical') means. The joke's humor operates via the logical consistency of what is observed, versus knowledge of what is actually occurring. This is an example of the fallacy post hoc ergo propter hoc.
Usage
This has led to phrases (and variants of) "You let the smoke out!", "The magic smoke is escaping." or "I think you set your power supply too high; there is magic smoke coming from that resistor!" as euphemisms for the destruction of a component by overheating.
When a device is powered on for the first time, it is often called a "smoke test".
Red Hat Linux 6.1 included the following text in the hardware compatibility guide:
Tier 3 Incompatible and Unsupported CPUs
The joke in the above quotation is the concept of a Smokeless CPU—one in which the vital magic smoke has been released, rendering it inoperable. The joke remained in Red Hat 6.2, but was removed in a subsequent version.