Other mistake: When Lucy is in the cafe watching the mother and her children, the mother repeats the same dialogue.
Other mistake: 18/2/15 In the live segment Tanya asks Jane "how's Adam" instead off "how's Ian" mixing up the names of character Ian Beale and actor Adam Woodyatt.
Other mistake: 14/07/11 - During Christian and Zainab's confrontation at Christian and Syed's flat, Christian is trying to gain Z's approval for his and Syed's adoption application. Throughout the entire 10 minute scene, the camera can be seen wobbling from side to side, as though all cast and crew are on the Titanic. The BBC even received complaints from viewers, stating that they had to turn the scene off because the camera angles were making them feel seasick.
Other mistake: 01/01/2013: James Branning's tombstone says he died on January 1st 2011, when he actually died on December 31st 2010.
Other mistake: 10/1/24 Sharon, Stacey, Kathy, Denise and Suki returned to the cafe to fill up the hole where they buried Keanu with cement, they manage to mix the cement and fill the hole without getting their hands or clothes dirty and left the cafe as clean as when they arrived.
Other mistake: Stuart drives to his wedding in the hearse, but he gets trapped inside when the battery goes flat, disabling the central locking and electric windows. Later on how does he manage to sound the horn if the battery is flat?
Other mistake: 25/12/17 Tanya is walking across the square to get Abi. It is raining heavily but Tanya is not getting wet.
Other mistake: Friday 12/8/11 Greg leaves for his holiday in Mexico by taxi. After he has gone you can see his pick-up truck parked in front of Dot's car in the square. On Monday 15/8/11, Greg's truck has gone.
Answer: Most likely a mistake by the set designers or it's a recycled set which already had the switch on (they just repaint and add other fittings).
At my workplace there is a two-gang switch with a three-gang switch underneath it, one of which is for the shop lights, the other four have no apparent use.