Other mistake: When Abigail keys in her password to the preservation room, she only presses the keyboard 9 times including the return key. If "Valley Forge" was the password, she would have had to have used at least 12 keystrokes (including the return key.) And she never goes near the "L". (00:33:20)
Other mistake: When Gates writes down the numbers from the back of the Declaration of Independence he forgets one number. On the document there are the following numbers: 11-6-6, 12-18-7, 14-3-7. Ben however only writes down 11-6-6, 14-3-7 and then keeps on. (01:01:10)
Other mistake: When Gates enters the password to enter the preservation room he accidentally hits the Caps Lock key instead of the "a" key. (00:42:35)
Other mistake: When Ian is breaking in to take the Declaration of Independence his team uses a plasma cutter to cut through some metal. A plasma cutter of that size (fairly small and portable, but able to cut through metal of that thickness) requires a 220v power supply and a compressed air supply. I doubt there was a 220v power plug in that area, and I didn't see them carrying around any kind of compressed air tank.
Other mistake: When Nicolas Cage puts on the thumb cover, he lightly dabs it in that purple stuff so he can pull Abigail's thumbprint off the glass. The very next shot is a close-up of the thumb cover, which is now somehow covered in that purple stuff - even on the back of his thumb.
Other mistake: When Ben is threatening to throw the flare down into all the gunpowder, knowing what would happen when the fire strikes it, there are all kinds of sparks falling from the flare down into the fire anyway. Wouldn't one of those sparks would easily have set the powder off? (00:19:35)
Other mistake: The signature on the copy of the Silence Dogood letter is different than the one on the original. They should be the same if it was truly a scan.
Other mistake: In the scene where Ben, Riley, and Abigail arrive at Patrick's house, the location title at the bottom of the screen says, "Philadelphia, PA." Then, as the Declaration of Independence is being examined, Patrick says that he donated the Silence Dogood letters to "The Franklin Institute in Philadelphia." Finally, as the scene ends, Ben, Riley, and Abigail are in Patrick's car, driving through the night towards Philadelphia. (00:55:55 - 01:04:15)
Other mistake: When Ben enters the Gala dressed in overalls and tool belt, he enters a door across the street from where Riley is parked. There is a security guard outside that door who waves Ben on into the building. As Ben exits that same door, there is no security guard, which seems hard to believe with such a big event.
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