Continuity mistake: When Riley is sitting in the car waiting for Ben, the window is gone and shards of glass can be seen in the window frame, but there has been no shooting yet. Later, the window is back and then it gets hit by a bullet and shatters.
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.
Factual error: On the back of the Declaration of Independence when the Masonic square and compass are revealed, there is a "G" in the center. This "G" was not used until the period of the Civil War. Confederate Masons refused to use the "G" as they considered it a Yankee trick. Even today the Grand Lodge of England does not use the "G." For these reasons it would be unlikely that the Masonic symbol displayed on the Declaration would have included a "G."
Continuity mistake: The school boy is counting letters on the Dogood letters and runs out to give them to Riley. As he crosses the street, he passes a cab and a Taurus station wagon. On his second trip out, he passes the same two cars again.
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.
Continuity mistake: When Ben is trying to grab Abigail from Ian's vehicle, we see a bus coming. There is a car on the right side of the bus (Riley's left side) in that shot. We then see Viktor stepping on the brake and Riley start pulling to the left of the bus; when Riley swing around to that side to miss the bus, the car has disappeared.
Continuity mistake: When Gates and Riley are talking in front of the Lincoln Memorial, the clouds in the background change. It would be possible that they change so quickly during the shots, but only if there was a gale and not sunny weather as it is in the movie. (00:27:00)
Continuity mistake: When Riley is first looking at the special glasses the lengths of the stems of the glasses change from shot to shot. Sometimes they are halfway extended and sometimes they are fully extended.
Continuity mistake: When Riley is in the Subway to install the video feed, he places his laptop on a table. On its left side there is a little antenna. When the laptop is on the ground, the antenna is horizontal. In the next shot, the antenna is already folded out in a 45° angle. (00:30:05)
Continuity mistake: When they blow the back of the Declaration with the hairdryer, Ben writes down the numbers. When he explains to Reilly how an Ottendorf cipher works we see that there are 2 columns with 17 lines each. This leaves us with a possible 34 letters. However the riddle that they decipher from the Dogood letters have 89 letters in it. After Ben writes down the last letters Abigail switches off the dryer and we can also see that it was the end of the page so there is no way that there was more space to get more letters.
Continuity mistake: When the FBI storms into Ben Gates' apartment, an agent is researching the Silence Dogood letters on his Hewlett Packard laptop. However, the web browser displayed on his laptop is the Mac Safari browser. HP laptops don't run Mac software so the "photoshopped" shot of the web browser on the laptop was done on an Apple Mac. (00:57:20)
Continuity mistake: At the Charlotte down in the cargo hold when Ben has just figured out that the riddle refers to the Declaration of Independence, Ian's stocking hat keeps changing between scenes. He's got a two-toned hat with a jagged edge between the two colors. Sometimes, the edge is turned up so far you can't even see this design. At other times, the edge is barely turned up and crooked and the jagged edge between the colors is very prominent.
Factual error: In the scene under Trinity Church, Ben is holding on to the elevator and trying to get a grip to swing to safety (after Dr. Chase is already to safety). He grabs a board that falls off, then the last board pulls back an is shown to be held on with modern day nails. If it were built 200 years before, as the movie suggests, these would not likely be the kind of nails from the local hardware store as shown in the film.
Character mistake: At the gala, Ben describes to Abigail how the Founding Fathers would have been hanged, beheaded, drawn and quartered, and had their entrails burned had they lost the war. However, only hanging and beheading were acceptable practices in Britain at the time; Ben's information is off by several decades.
Continuity mistake: When Dr. Chase is entering her password she goes for the first letter on the keyboard. Her little finger is at the right end of the space bar and her index finger is hitting the key. When the shot changes to the close-up of her hand, the little finger is in the area of the CTRL key and the middle finger is the one which hits a key, in the area of the m or comma key. (00:33:15)
Continuity mistake: When Ben is in the Men's restroom taking off the overalls to reveal his tuxedo, he is in front of the third or back sink. He turns, throws tool belt away, turns back toward mirror and grabs his drill and starts to leave with the red NO SMOKING sign on the mirror up to his left. Then he stops to look in the mirror again as Riley says "How do you look?" He is looking into the mirror straightening his tie and the red NO SMOKING sign is on his right. The next frames he is back in front of the third sink smoothing his hair back and the NO SMOKING sign on his left.
Continuity mistake: The Declaration of Independence in the clear plastic cover keeps changing colors from brown to white and back. When Ben puts it in the very first time while in the elevator, it is brown. When Ian gets the case out of the street after Abigail loses it, it is white. It is back to brown in the church.
Continuity mistake: When the guy is smashing Parkington Lane's headstone, the number, size, and location of the holes change from one shot to the next. (01:39:10)
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)
Factual error: When Riley is sitting outside the Franklin Institute and sees the ad on the side of the bus with the close up of the Liberty Bell, the SEPTA bus is labeled with the 108 Airport route. That bus route goes from the 69th Street Transportation Center to the Philadelphia International Airport and would not pass near the Franklin Institute.
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