His Last Vow - S3-E4
Factual error: Sherlock takes pains to explain in detail that he will deliberately corrupt the magnetic code stripe on an access control card he intends to use to enter the elevator to the villain's penthouse, by carrying it next to an operational cell phone. (This is possible due to the low frequency magnetic field from a phone's vibration motor.) But when he actually goes to access the elevator, he simply touches his access card to the reader instead of swiping or inserting the card through a slot, which is how a magnetic stripe reader would operate. The elevator uses an RFID proximity reader, not a magnetic stripe reader - a phone wouldn't corrupt an embedded RFID tag.
Factual error: Will's hospital chart states he is taking Warfarin and Coumadin. These are both the same drug to prevent blood clots. He would not be on both. (00:31:50)
Factual error: At Walter's apartment, Annie notices a group of spy novels on his shelf. The "spy novels" are actually six books from the Goosebumps series by R.L. Stein, The Day My Butt Went Psycho by Andy Griffiths, four books from Lemony Snicket's A Series of Unfortunate Events, and the British covered edition of Harry Potter and the Prisoner of Azkaban by J. K. Rowling. (00:18:30)
Factual error: Emily is supposed to meet Paige at a karaoke bar at the intersection of 202 and W. Chester Pike, however this intersection is actually an overpass. 202 crosses over top of West Chester Pike, but never actually intersects it.
Factual error: Throughout the series and credits, the ranks of Detective Superintendent and Detective Chief Superintendent are abbreviated as DSU and DCSU instead of the correct DSI and DCS.