While opening a Mac Mini 2010 I discovered that the insulation on the short wires connecting the AC-In socket to the power supply was cracked. The "twist" operation to remove the AC-In socket from the case caused the wire insulation to crumble to small pieces. In the end, all the insulation fell off, the wires remained completely bare.

Mare sure the Power Supply is not connected to mains AC power while working on it.

Discharge the Power Supply before touching the wires by pushing the power button while the AC cable is unplugged then shorting the AC terminals in the plug with a screwdriver.