Power Surge, Reversed Polarity, Power Supply plugged in the wrong way around: This is a frequent occurrence - particularly plugging the power lead in the wrong way around.
Despite the shape of the Molex connector, the cheaper versions are often are made of a soft plastic and can easily be plugged in incorrectly.
This is especially true if you try to plug a laptop hard drive in a USB enclosure or if you plug the laptop hard drive to an IDE-to-laptop connector incorrectly. You must make sure you do the right thing as the outcome of your mistakes can be disastrous.
Similar faults occur when the power surge can damage the computer motherboard and hard drive circuitry. In this case, you must immediately unplug the computer from the mains power supply and withdraw the hard drive to a safe place until data recovery can take place.