When investigating the 20-year old murder, they are attempting to figure out who Melanie was having an affair with. It is explained that she had two male co-workers who were married to each other. Gay marriage was not legal in New York until 2011, twenty years later.
Castle explains that the magnetic field generator would have "knocked out the video and explain the moving tripod on Jack's video" But that tripod is an inexpensive model made of nonferrous metals and plastic. Magnetic fields would have no effect on it.
Ryan says the female murder victim worked with two men who were married to each other twenty years earlier. Same sex marriage was not made legal in New York until June 24, 2011, the same year the show aired so it would be impossible for two men to be married to each other in 1991, twenty years earlier.
Castle talks about Scotch castles. Scotch is the whiskey made in Scotland. The people are Scots not Scotch and the correct term is Scottish castle.
As Castle describes a scene in which the victim observes a murder in 1991, the flashback shows a young boy near the crime scene. In the foreground is the back end and taillights of a Ford Crown Victoria of a body style that wasn't developed until a decade later.