I'll start with one from a friend of mine (who shall remain anonymous): he lives in a place with old 120V electrical wiring. He put a rig of ~1.6kW (~13A), as measured by a kill-a-watt, on a 20A circuit for about half a day, until he started smelling smoke in his apartment, apparently coming from the wiring inside the walls. He completely stopped using this circuit, and now runs his rig on a dedicated 240V circuit. A 20A circuit is normally rated 16A for continuous loads by the National Electric Code, but that old wiring was likely defective and the insulation probably started melting.
Did you actually measure the current with the Kill-a-watt? Most my computer have a power factor of about 0.67. That means that for every 100W (833mA resistive), the actual current draw is closer to 1.24 amps. or in your example, 19.9 amps. My kill-a-watt is only rated to measure up to 15Amps, IIRC.