The Egyptians did not have any concept of PI until over 800 years after the pyramids were built and when they did their calculation was (incorrectly) 3.16. The reason why PI seems to pop up in certain ratios of measurements of the pyramids is most likely because they could have used wheels as measuring devices.
Napoléon Bonaparte speaks with a French accent. It is known that he never had one to speak of, always speaking French (a second language of his) with a Corsican accent, which sounds closer to an Italian accent.
Custer is portrayed with a southern (perhaps Virginian) accent, but he was from the Ohio-Michigan region. He introduces himself as "General Custer of the 7th U.S. Cavalry," but he was a general only in the 3rd Cavalry, and a lieutenant colonel in the 7th. Finally, the number of Federal soldiers who died at the Battle of Little Big Horn was 268, not a mere 208 as Custer claims.
Larry tells his son that the Smithsonian Institution is a collection of 19 museums, which is true. However, he also says that they are all located on the National Mall. Some are actually located in New York City, Chantilly in Virginia, and different parts of Washington, D.C. The National Zoo is also part of the Smithsonian group of museums. The National Gallery of Art, though located among the Smithsonian, is not part of the Institution, contrary to the implication in the film.
The WWII Tuskegee airmen are wearing modern USAF flight jackets with Velcro. Although Velcro was invented in 1941, it was not patented and in common use until the 1950s. The same jacket has a 50-star flag patch, but there were not 50 states until 1959, 14 years after WWII ended.
When Larry first arrives at the Air & Space Museum he sees the twin seat, twin canopy Air Force jet with both aircrew in the cockpits looking at him. Shortly after the aircraft is shown facing the other direction (no aircrew are in the jet) and one is walking on the ground below the aircraft. Not enough time passed for the aircrew to get out of the plane nor for Larry to change his visual perspective of the jet.
In the first scene that General Custer is announcing his attack plans and asks if there's any questions, he tries to pronounce Sacajawea's name the first time, he is clearly standing on a crate above everyone else. The second time he tries, he is at the same level of everyone else. On his final attempt to pronounce her name, he's back on top of the crate.
When Sacajawea is placed in her crate during the opening credits, she is laying in curly straw, with no white Styrofoam surrounding her. The lid is then put on the crate, however when she bursts out of the crate white Styrofoam is seen exploding out as she lifts the lid.
When Custer is announcing his plan to attack when he yells "attack!" you can see Attila the Hun behind him in a corner of the crate. As he tried to pronounce Sacajewea's name, Attila is behind her. Then, as Custer sits back down, Attila is once again in the corner by Custer.
As Larry is walking towards the Smithsonian Castle, you see his cell phone on his belt. As he is walking by the Al Capone exhibit near the entrance, his cell phone is gone. Then, as he approaches the Kahmunrah gate, his cell phone is back on his belt.
When Larry explains to Amelia why he left the job as a night guard, he places the tablet on a pillar near the Abraham Lincoln statue. When he is arguing with Lincoln, the tablet disappears, but then they leave and Larry is holding the tablet again without picking it up.
Throughout Kahmunrah's conversation with Oscar the Grouch and Darth Vader, a Russian soldier is standing behind Ivan the Terrible's shoulder. However, when the camera flicks back to Kahmunrah as he waves goodbye to Oscar the Grouch and Darth Vader, not only has the soldier disappeared, but visible in the background is Al Capone whose skin is clearly not black and white as it should be.
When Larry and Kahmunrah are arguing and Kahmunrah is trying to get the tablet from Larry, and Larry is trying to get the hour glass from Kahmunrah, there are points when no sand is pouring through the hour glass, where according to the position of the hour glass, there should be sand flowing through.
The quick cuts between scenes in the Air and Space museum, the White House, and the Abraham Lincoln Memorial imply that they are adjacent. However the White House is 1 mile away and the Lincoln Memorial is over 1.6 miles away.
Grant Wood's "American Gothic" and Edward Hopper's "Nighthawks" are at the Art Institute of Chicago. Auguste Rodin's "The Thinker" is at the Musée Rodin in Paris. "Venus Italica" is at the North Carolina Museum of Art. Edgar Degas's ballerina is at the Royal Academy in London. Jeff Koons's "Balloon Dog (Red)" is not part of any museum's collection.
Larry and Amelia Earhart take off in the original 1903 Wright Flyer. This Flyer is only capable of carrying one person. Although they fly it using a stick control, it was actually controlled using a mechanism attached to the pilot's hips whereby he could turn the aircraft by shifting his body from side to side. Also, it no longer hangs from the ceiling of the National Air and Space Museum, but is in a second floor exhibit hall sitting on the floor.
When Amelia tells Larry that he couldn't keep his "cheaters" off her "chassis" (her figure) she probably meant "peepers" off her "chassis" as cheaters was period slang for glasses and peepers was period slang for eyes.
When the giant octopus jumps into the reflecting pond in front of the Abraham Lincoln Memorial, it shows him jumping into a pool deep enough that he has to float up a bit to have most of his body above the surface. In fact, the pool is only 30 inches deep at the very center of the pool.
The light on top of the Capitol is first shown as on when Larry and Amelia race to the Air & Space Museum. In later shots it is off. The light is supposed to be lit only on the rare occasion when the House or Senate are in session at night.
When Larry and Amelia are cornered near the American Gothic, Larry grabs the pitchfork from inside the picture, but he is shown using a four-pronged pitchfork. The pitchfork in the actual painting is a three-pronged one.
When the Wright Flyer is shown in closeup (with Amelia and Larry flying the plane) you can see that the engine is mounted mirrored to the correct position. The intake and exhaust of the engine was on the pilot-side, not the crankshaft.
The Wright Flyer and the motorcycle both start and run, showing that they have gasoline. All machines are drained of fuel and the tanks cleaned before they are put on display due to fire danger. Therefore, without fuel, no engine in the museum would run.
As George Foreman says "That just two years ago you worked as a night guard (etc.)". The First Night at the Museum came out in 2006 and two years later from that would be 2008, even though this one came out in 2009. Making it 3 not 2.
When talking about pi, the Albert Einstein bobble heads describe it as: "3.14159265, to be exact". Although the real Einstein would know that pi has infinite decimals, his bobble heads are giving the new combination to Ahkmenrah's Tablet, which is an exact number.
Dr. McPhee has no reason to allow Larry to stay in the museum after closing, as Larry is no longer a night guard. However, Dr. McPhee may feel indebted to Larry after the events of Night at the Museum, and decide to owe him this special favor from time to time.
When Jedediah is threatened with burial in sand, it is not the same threat as to a normal person. He is not a human who can suffocate but a toy figure who returns to life each night in presence of the tablet. However, some damage to him is certainly likely regardless of that fact, so the threat is still effective.
Larry needs Brandon/Brundon's security pass so he can move around the Smithsonian, and is shown using it on several occasions, yet Kahmunrah's gang are able to roam about freely. This could be a perk of being magically bound to the Institution.
Although George Armstrong Custer is typically portrayed with long hair and referred to as General, he had been reduced to his permanent rank of Captain after the Civil War. He was a Lieutenant Colonel at the time of his death at Little Big Horn - and had also cut his hair short to protect himself against being scalped. However, it's apparent that here he's portrayed in his Civil War "glory days" - he has a major general's shoulder boards and his double-breasted frock coat has the correct number of buttons. Additionally, he has his trademark red scarf and pin he wore on it at the time. Finally, he's portrayed as the 20-something he was 12 years before his death in Montana.
When Custer is knocked off the bike, he holds a hand up to his nose, indicating that it may have been broken, and, finding out it wasn't, uncovering it. It could also have been that he was "fixing" it, knowing it would be fine and not need extended healing.
Octopuses are ocean creatures which can only live in salty water, but this is a magic octopus which can breathe air in the museum corridors and saltless water in the Washington Monument Reflecting Pool.
When Larry opens up the octopus crate the octopus breaks out, which demolishes the crate, but not before. Later we see him brake crates with just one of his tentacles but he couldn't break his own until Larry opened it.
It is established from the previous film that all the museum characters are not their actual historical counterparts, and do not possess the same abilities or even the same personality, as Teddy Roosevelt explains at the end of 'Night at the Museum' (2006). Therefore, they are not likely to have any knowledge of themselves historians do not know, yet, this is completely overlooked in this film.
The first Night at the Museum established that if the tiles aren't perfectly flat on the tablet the magic didn't work. When Larry throws the tablet to Amelia Earhart, the tiles are obviously not flat but the magic of the tablet remains.
When the Wright Flyer is flying in the National Air and Space Museum, the timing chain is moving. While outside the chain has stopped moving, although the propellers are still turning. There is nothing in the plot to indicate that they ran out of gas.
Jedediah is very small, appearing to be no more than two inches tall. When he is in the hourglass the sand is the incorrect size. To be in proportion to him the sand grains should be almost as large as a basketball.
The goof items below may give away important plot points.
When Kahmenrah calls first his army from the underworld, he uses the same language he used to command his own Egyptian followers, to kill Larry, and they understand them perfectly. Yet, when Abraham crashes in after Octavius to save Larry, Kahmenrah has to caw like a bird to command them, as they don't seem to understand him.
When Amelia lands her plane in New York at the end, you can see Attila the Hun and others exit the plane, but not Sacajawea. But soon after, she is seen among a group running up the stairs to the museum, having come out of nowhere.
Amelia Earhart's red Lockheed Vega carries no more than two people (not the entire museum crew) and could not make a Washington-New York round trip in a single hour before sunrise. Real trains take three hours and jet aircraft take almost an hour, one way. The fantasy parameters established by the storyline do not provide any explanation as to how the plane's capacity and speed could exceed those of a 1930s machine.
Night at the Museum explained that at sunrise, anything that gets left outside the museum after being brought to life turns to dust. In this one, the statue of Abraham Lincoln is in the open-air Memorial, but since that is his authorized space, he is considered to be "inside."
When Jedediah is in the hour glass, he is in the lower chamber, and the area between is too small for him to go through. When the minions are returning to the portal, in a close up he is obviously in the top chamber, only to return to the bottom when Octavius breaks him free. Some might say that the hourglass could have been turned, but that would obviously have caused him to suffocate if it weren't already an established fact that he could shift his weight around enough in the case of a turnover to be able to continue to breathe.