Factual error: When Cindy wanted a hitman to kill Ian she looks in the yellow pages to find one. Surely the police and the makers of the yellow pages would have been alerted if a hitman agency wants to advertise itself in the yellow pages?
Factual error: When Barry dies in the Scottish Highlands, Janine has him cremated. However, she manages to get back to Walford in roughly 4 days, no police force would allow for a body to be cremated 4 days after death, especially with an unknown circumstance of death. Also it would look very suspicious that the widow was so keen to get her husband cremated, slowing the police force down even more.
Factual error: In the 2008 Christmas special, Shean fell through the ice on a frozen lake. The water would have been jet black once below the surface, and the water temperature, just above zero, would have meant near instant muscle spasms followed by muscle paralysis, making swimming around and holding your breath completely impossible.
Factual error: 7 Sep 2015: Several errors with the soldiers in the recruiting office. They wear their berets almost flat on their heads and not pulled to the side - no self-respecting British soldier would wear his beret this way. The berets have tails, which British berets do not have. Lee salutes a sergeant major - he's an experienced soldier so would never do such a thing and would get an earful if he did. Lee talks about his platoon in Afghanistan - he's in the Royal Engineers, who have troops instead of platoons. In fact, all the soldiers are from the RE - in reality recruiting offices are staffed by soldiers from a mix of corps and regiments.
Factual error: In the episode where Kat is flying off to Ibiza, she is seated in an over-wing exit seat. These seats are reserved for ABP's (Able Bodied Passengers) which mean no children or infants under the age of 14 can sit there. Once the plane has started boarding, check in closes for that flight and no further tickets can be sold for that flight. The captain requested pushback, but, certain conditions around the aircraft wouldn't make this possible. Stairs still in place, ground power was attached and once the door had been closed Alfie would have not under any circumstances been allowed onto a airfield to climb the stairs. If he did, he would have been escorted off.
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.