This is one reason why use of compressed air on boards is required to remove carbon buildup, debris, or even wire whiskers, especially around capacitors. It can act as a conductor, develop a a short circuit, and blow up. Every routed game should have this done before servicing the boards. However, do not sustain the air on the board, unless using a electric blower, as the cold compressent can damage electronic components.

The other most common reason is reversing the capacitor lead terminals when rebuilding a board. It has the same result.

It is generally wise to do voltage regulation testing before plugging in primary controller PCBs. This did not occur.

Also, always recheck and inspect work before repowering as well.