Continuity mistake: When the troop is flying in the helicopter at the start of the movie, Epps is not wearing sunglasses. When it cuts back to him, he is, then again it cuts away and back several times and you can see an indentation on the side of his head where the Oakley sunglasses were.
Continuity mistake: In the opening attack scene on Soccent AFB, Colonel Sharp pulls an axe off the wall to literally cut the server hard lines. This axe was not on the wall in the earlier dialogue between Colonel Sharp and the one who was telling him about the unidentified aircraft. (00:07:40)
Continuity mistake: When the SUVs arrive at the Hoover Dam, the lead car pulls up to the curb and the doors open directly in front of the first elevator tower (there are four, which are seen in overhead shots), on its right, with the two Nevada intake towers well ahead of the SUV, on its left. Next close-up, however, as Sam exits the SUV the vehicle is now parked farther up the road, directly in front of the Nevada intake towers to its left and closer to the second elevator tower. Note the yellow pedestrian walkway markings (leading to the Nevada intake towers) the SUV is parked on in the second shot. (01:34:25)
Continuity mistake: When Sam grabs his mom's pink bike, there is nothing in the white basket at the handlebars as he flees from Bumblebee. However, at the start of the shot just before the tire hits the broken concrete, a square pink pillow suddenly appears in the basket, before flying out as Sam flips over. (00:48:10)
Continuity mistake: Before Frenzy goes to the POTUS mainframe, aboard Air Force One, in the overhead shot of the cabin the seated Secret Service agent holds in both hands the unopened USA Today, with the back's weather page facing out. Next shot, the newspaper is open in his hands. (00:28:10)
Continuity mistake: When Sam kicks Frenzy like a football in the wideshot, the right side of his boxers are torn straight up the leg, but are intact thereafter. (00:54:40)
Continuity mistake: When boxer wearing Sam begins to run away from Frenzy, in the shot facing his legs from behind he is wearing blue socks with black stripes, but then it changes to blue socks with white stripes. (00:54:00)
Continuity mistake: After Sam knocks Mikaela off her moped, they are both on the ground right beside the moped when Bumblebee shows up, but in the overhead shot as Sam helps Mikaela up the moped is gone, not moved, just gone. (00:51:15)
Continuity mistake: In the scene where Sam and Mikaela are looking at Bumblebee's engine, both of Bumblebee's doors are open when Mikaela starts walking away. Sam jumps in the car and pleads with it to start, and the shot changes to side view from passenger side of him trying to start. Here both doors are open as well. But when the shot changes to Bumblebee starting up and Sam jumps out to close the hood, the passenger door is suddenly closed.
Continuity mistake: When Optimus Prime first arrives in Mission City, he knocks over a few garbage cans, and a few pieces of paper gets stuck on the grill of the truck. But in the next shot, the pieces of paper are gone. This is not because he slowed down, because the grille is out of shot for less time then it would take paper to fall off. Not only that, but the air blowing at the paper would make sure it sticks on. (02:00:00)
Continuity mistake: At Bobbie B's, when Sam first opens Bumblebee's driver-side door, the gas pump's white steel guardrail is directly behind him, aligned with the driver's seat, but when all the windows shatter, Bumblebee's front fender is aligned with the guardrail, though Bumblebee has not moved. Same goes for the VW Beetle beside the Autobot. (00:14:00 - 00:15:10)
Continuity mistake: When Ron and Sam, followed by Bumblebee, make the U-turn in front of Bobby B's and then park, the spinning 'Bobby B. sez...let's talk deals' sign is to the left of the three vintage gas pumps, near the clown. When Bobby B. puts his arm around Sam's shoulder and says, "Sam, your first enchilada of freedom..." the spinning sign is now on the other side of the pumps, where there wasn't a spinning sign previously. (00:12:10 - 00:13:15)
Continuity mistake: When Ron and Sam park in front of Bobby B's, there are three cars parked in front of the three pumps - a white vinyl top, a wagon, and a red car. When Sam says, "You said half a car, not half a piece of crap, Dad," the red car has been replaced with an old blue Plymouth, specifically for the "50-year-old virgin" joke, because the red car is back in following shots. (00:12:20)
Continuity mistake: Before Barricade interrogates Sam, the Decepticon throws Sam on the windshield of a car, and a wheel falls off the car's roof, but then reappears back on the roof in a following shot.
Continuity mistake: After Mikaela hacks away at Frenzy, when Sam kicks him like a football he uses his right foot in the close-up, but his left foot in the wideshot. (00:54:40)
Continuity mistake: When Barricade interrogates Sam about the glasses, the cracks on the shattered glass windshield under Sam disappear, reappear and change characteristics between the interior (inside the car), overhead and close-up shots. (00:50:35)
Continuity mistake: After Sam is knocked off the pink bike, he ends up on the ground as Barricade, aka the police cruiser, drives forward causing Sam to keep backing up, which leaves the bike near the back of the cruiser. Yet, when Barricade threatens Sam with its weapons, the bike is right beside the cruiser's front end again. (00:49:35)
Continuity mistake: In the scene where Sam is being interrogated at the police station, the cop catches the drug bottle with the bottom facing up. When it cuts, the top is facing up. (00:38:20)
Continuity mistake: Aboard Air Force One, when Frenzy kills the first two Secret Service agents, the second agent falls back into the hallway, but his feet are still in the room over the bottom raised lip of the doorway. When the third agent fires on Frenzy, the legs of that second agent are lying flat on the room's floor, right beside the first agent that was hit. Note that the agent's legs are also beside the shelving unit, which is a good distance from the door. (00:31:45)
Continuity mistake: When Mikaela looks under the hood and fiddles with Bumblebee's distributor cap, her long hair is either behind her shoulders or in front, depending on the angle of the shot. (00:25:15)
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.
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