When attempting to repair the electricity supply, Joan comes down to talk to Sherlock about Mycroft. Sherlock says 'by all means, distract me while I'm holding 240 volts in my hand'. 240 volts is UK mains voltage. On the surface, Sherlock's statement would seem to be a goof, but in reality, 240 V exists in US Home Service Electric Power Panels. The majority of outlets / receptacles in the US Home are 120 volts, which is achieved using a split-phase power system. 240 Volts is brought into the home, with one line being 120 V above earth ground, and the other being 120 V below ground (actually 180 degrees out-of-phase with the other 'line'). Most circuits run between earth ground and one of the lines, which provides for 120 V service, hence the potential for an incorrect "goof." Larger appliances (electric stoves, clothes dryers, hot water heaters, air conditioners) run on 240 V, and have their circuits run from line-to-line instead of line-to-earth.