Factual error: During the heist preparation scene, Riley enters the National Archives Metro station to record surveillance camera footage. Surveillance camera feeds for the National Archives would never be accessible within a Metro station due to both security concerns and the fact that the feeds are not needed outside of the National Archives building. (00:32:00)
Deliberate mistake: In the two scenes when Ben and Riley are in the documents room, it is in full light. The rotunda is actually dimly lit as a way of protecting the documents from light damage.
Continuity mistake: When Riley is in the subway to get the video footage, he starts to record the preservation room that shows the declaration on the desk, but how can It be there if Riley hasn't used the laser to get it moved there yet?
Continuity mistake: When Ben and Ian get into an argument over stealing the Declaration of Independence, Ian says "all you're going to be is a hinderance." Shaw then points a flashlight and a gun at Ben. When the camera angle is on Ben, there is no flashlight beam visible on him. Then Ian says, "I'll shoot your friend", Shaw points the flashlight and gun at Riley, the light beam visible on his face. Ben lights a flare. Shaw points the flashlight and gun back on Ben, and again, no flashlight beam is visible on him.
Continuity mistake: After everyone gets off the elevator down under the church, Ben has the declaration of independence then Riley has it without Ben giving it to him.
Plot hole: The tunnel at the base of the stairs under the church wasn't connected to the stairs and seems to only have been reachable with the elevator system.
Factual error: The poem on the pipe stem reads Fifty Five in Iron Pen, and Nicolas Cage states "a document that fifty five men signed." The Declaration of Independence was signed by fifty six men.
Deliberate mistake: When Riley uses the video camera laser to disable the Declaration of Independence sensor, that could never happen since cameras are not permitted in the documents room.
Answer: He doesn't simply assume "Beneath Parkington Lane" means beneath the church: Parkington Lane is the name etched on the tomb hiding the entrance to the tunnels. When he saw it, he naturally deduced what he had to do.
Sereenie