Continuity mistake: At the end of National Treasure 1, Riley has a Ferrari 360 Spider. At the beginning of Book of Secrets, when his car is being towed away, it's a Ferrari F430 Spider. At the end of the movie, he is given back a 360 Spider.
Continuity mistake: During the car chase in London, when you see from inside the car, you can't see any of the glass at the bottom on the rear window, but when you see the car from the outside there is a significant portion of the glass left.
Continuity mistake: During the scene where they encounter the beer truck in the alley, they are driving towards it, but in the next shot the car is the other way and are driving away from it.
Continuity mistake: Mitch Wilkinson jumps to a hanging ladder in the hidden chamber at Mount Rushmore, the first shot shows him hanging off the ladder with a pistol holster through his belt loops. The shot cuts away and when it returns there is no pistol holster where there was seconds earlier. (01:36:00)
Continuity mistake: Many beer kegs fall from the truck, yet the number remaining on the truck barely goes down from shot to shot.
Continuity mistake: During the car chase in London, the Range Rover with Mitch Wilkinson crashes into the rear end of the Fuller beer truck, damaging the left front bumper parts and headlights. Later in the chase, no damage is seen.
Continuity mistake: In the final scene of the movie, when Riley is walking toward his car, he walks approx 30 feet from behind the car up to the car, notice that there are no other cars behind him. As soon as he starts the car he accidentally goes in reverse and we hear him crash into another car, yet there was no one behind him for at least 30 feet. We did not see or hear another car pull in behind him in that few seconds. There is no car parked behind him.
Continuity mistake: When Ben Gates and company first examine the Booth diary page, they notice a few of the digraphs relevant to the cipher. After he says to flip the page, pause it and notice the digraphs that are visible. They are: ME IK QO CQ TE ZX. Later, when Ben discovers that the key to decoding the cipher is DEATH, Riley inputs the keyword into his computer. Pause it when it shows his computer screen. The various digraphs that they have input are nowhere close to the ones they discovered on the page. They are: QF JU WJ AK EH. Not even the same amount of digraphs, no less. The results are larger too. The entire interface is different for the two shots.
Continuity mistake: When Ben and Abigail are examining the Resolute desk at Buckingham Palace, Ben starts opening the drawers and discovers the code option numbers on the underside of the drawers for the first time. When he does this, in the shots of the underside of the drawer, Ben is pulling the drawers out almost all the way, but in the shots from above, he is barely pulling the drawers out at all.
Continuity mistake: In the car chase in London, when Ben throws the plank into the Thames the Range Rover is missing the bumper and stops when knocking over a roadworks sign. The next shot shows the car far from the sign, with the bumper perfectly back to normal.
Continuity mistake: In the first film, John Adams Gates (Christopher Plummer) tells his grandson, young Ben, about how a boy named Thomas Gates learned about the Templar treasure from Charles Carroll in 1832. John refers to Thomas as "my grandfather's grandfather," making him Ben's Great-Great-Great-Great-Grandfather. The adult Thomas Gates is in the prologue of the sequel deciphering Booth's diary in 1865. His son, Charles, is said to be the grandfather of Patrick Gates, played by Jon Voight. That would make Thomas only Ben's great-great-grandfather, which another character refers to him as later in the second movie, which in turn makes Thomas Gates just the grandfather of John Gates. Either John Gates' memory is fuzzy in his old age or two generations of Gates men disappeared from the timeline. Perhaps John meant to say it was Ben's grandfather's grandfather, which is how Thomas is related to Ben in this movie.
Continuity mistake: Just after Ben mentions Montesquieu to the French policeman, the other policeman's head changes position between shots. (00:21:55)
Continuity mistake: During the London car chase scene, Nicolas Cage throws the wooden artifact from the car into the river. The bad guys in the Range Rover then stop, colliding with some roadworks, although in the next shot the car is suddenly a few metres away. (00:42:30 - 00:43:00)
Answer: It is never explained what was on the page. It was just a cliffhanger to set up for a 3rd movie, I think.