Continuity mistake: When Bumblebee is caught, so are Sam and Mikaela. They were on the ground being handcuffed. But then Sam gets away and starts running for Bumblebee to stop him from being frozen. Sam is no longer handcuffed at this point.
Continuity mistake: When Mikaela looks under Bumblebee's hood, in close-up shots, her tummy is shiny and oily. In other shots, it is completely dry. (00:25:25)
Continuity mistake: When Frenzy is attacking boxer wearing Sam at the fence, Sam's right sleeve at the elbow gets torn while fighting the little lunatic. That large hole disappears when Mikaela arrives with the DeWalt Reciprocating Saw. (00:54:20)
Continuity mistake: When Sam, who is wearing boxers, is running from away Frenzy there is a large tear on his left sleeve, at the elbow. When Sam reaches the fence Frenzy attacks him, and as Sam covers his head with his arms we see the tear is gone from his left sleeve (viewer's right). (00:54:05)
Continuity mistake: After Sam knocks Mikaela off of her moped, he tells her to get in the car, and she climbs onto the passengers seat, while Sam get into the drivers seat. But in the next shot, there is no one visible on the passenger side of the car. It is best visible just before the lens flare. (00:51:40)
Continuity mistake: At Hoover Dam, when the soldiers salute Keller the clock reads 10:00, then as Keller, the soldiers, Maggie and Glen walk away in the background, when Simmons tells Sam that they got off to a bad start the clock reads 11:35, and next shot when Banacheck interrupts it reads 11:43. Also when Simmons talks to Mikaela and Sam the word "Nevada" is gone and it only reads "Time" under the clock. Then when Banacheck says, "Come with me, we'll talk about your car," the word "Nevada" is back under the clock. (01:35:25)
Continuity mistake: After Frenzy uses the POTUS mainframe aboard Air Force One, when he kills the third agent the man falls backward towards the back wall, but in the next shot as the other agents arrive, the body faces the opposite way. (00:32:05)
Continuity mistake: At the park, when Sam decides to drive Mikaela home, Miles is in the car with his right arm (viewer's left) up behind his head, but in his close-ups his arm is down. (00:22:30)
Continuity mistake: In class for his genealogy report, when Sam begins to dump his great-great-grandfather's belongings onto the desk, while the backpack is upside-down it turns around from front to back, between the wideshot and next close-up. (00:08:55)
Continuity mistake: When Sam's parents begin knocking on his bedroom door, Prime says, "Ratchet, point the light," and nothing is on the bathroom windowsill, but when Ron is searches Sam's bathroom with the flashlight there are two small overturned flower pots lying on that windowsill. The flower pots are gone again when Ironhide asks, "The parents are very irritating, can I take them out? (01:14:05)
Continuity mistake: When Prime lifts Mikaela up to Sam's bathroom window, there are two baseballs in acrylic display cases and another object (looks like a vintage camera) on the windowsill (beside the tub). Then when Sam runs to that window and sees the Autobots parked as if it were a "truckstop", all those items disappear and are replaced by two small overturned flowers pots. (01:11:55)
Continuity mistake: When Scorponok's tail is being examined aboard the transport plane, as it curls itself to strike, Epps jumps back with his hands up near his head, but next shot Epps's hand is positioned on the table for the dramatic near miss. (01:06:50)
Continuity mistake: When Sam and Mikaela are in the SUV with Simmons, the look of the backseat of the vehicle, between the two teenagers, changes when Prime appears, and provides them with his version of a sun-roof.
Continuity mistake: After Sam takes off running with the All Spark, heading for the building, Prime is shown driving down the street towards a collection of garbage bins. In the next shot, before Prime hits them, all the garbage bins are set up very differently. (01:59:45)
Continuity mistake: While standing in front of a frozen Megatron, after Banacheck explains that Sam's great-great-grandfather made one of the greatest discoveries, Sam and Mikaela are shoulder to shoulder, but in the next back shot they are 4-5 feet apart. They're beside each other again in following shots. (01:37:20)
Continuity mistake: The rather large hole at the back of Sam's long and short-sleeved tee shirts, that appears after Barricade interrogates him and he knocks Mikaela off her moped, changes size, position etc. There are other small rips around that hole that appear, disappear and change as well.
Continuity mistake: When Mikaela has Bumblebee at the back of Mike's Towing truck, she drives down an alleyway, and when she stops the truck in the close-up there is a trashcan and other debris between the front two tires, which vanish in the wideshot. (02:02:25)
Continuity mistake: When Agent Simmons takes Mikaela and Sam into custody, the headrests in the SUV vanish when Prime shows up. The driver's headrest is visible just after they enter the car. When Sam says, "My car had been stolen, from me, from my home," and when Simmons asks, "What do you kids know about aliens?" the headrest behind Sam is visible, among other moments. (01:19:00)
Continuity mistake: When Sam is being chased by Barricade, he is thrown on the windshield of a car and then mutters about it being a bad dream. In the next shot, a wheel and other junk has suddenly appeared on the car's roof. (00:50:35)
Continuity mistake: At the Pentagon, when Maggie is sitting with her team of 'experts' and one suggests Iran, in the shot facing Maggie's back her file folder is closed and lying on the table in front of her, but in the next shot, she closes the open folder. Between these two shots, the analyst to her right goes from holding his headset by his ear, to holding a pen on the book in front of him. (00:27:50)
Answer: Because the Decepticons didn't know where Megatron ended up for the last 100 years. Bumblebee arrived on Earth as a scout to check out for any clues, found the glasses on Ebay, signalled the other Autobots, which prompted Blackout and Frenzy to investigate the base in Qatar, which lead to Blackout downloading the base computers and Frenzy hacking the computers aboard the aircraft later leading to the discovery of Megatron's whereabouts.
GalahadFairlight