Sometimes when working on electronics, especially computers, something will short out and you will see and smell a dark smoke come out of your electrical device. That is magic smoke, and once it leaves your device it is gone forever. You need that magic in order for it to work properly.
If that happens you have a few courses of action. Remove it from the power source if safe to do so, such as unplugging it from an outlet. Using a multi-purpose fire extinguisher or fleeing the building to safety then calling the fire department.
If you don't already have a fire extinguisher, know where it is, and make sure it hasn't expired, you should put that on your to do list.
Note: This is only my advice. You may have legal or contractual obligations to report any or all fires as per a home owners or renter's agreement. Even if you unplug the device and your outlet breaker doesn't trip it doesn't mean it didn't cause damage to anything else, or cause a fire somewhere along the electrical system of your building. Consult a professional if you have any questions.
Photo Credit: Flikr User K. Latham, Source