Continuity mistake: After Megatron says to Optimus: ''Then you will die with them!'', Megatron throws Optimus away, and the latter one heavily damages a car, and two others skid sideways in order to brake in time. Two other cars remain parked. But in the shot where Optimus gets back up and draws his gun, only one car remains. Even if the two cars that stopped just before they hit Optimus drove away, there would still be two parked cars and one heavily damaged one. (02:01:00)
Continuity mistake: When Optimus is holding Sam and Mikaela during Sector Seven's search for them, Mikaela falls, and in the shot facing up Sam hangs onto Optimus with his right hand and grabs her with his left hand. In the next overhead shot Sam now hangs onto Optimus with his left hand and holds Mikaela with his right hand. (01:26:10)
Continuity mistake: When Simmons leads the way into the room with the old Army radio console, he pushes open the massive doors with Keller and the rest behind him (we see the very distinctive wall opposite those doors). When Frenzy pays them a visit at those doors, just as Simmons shouts, "Burn, you little sucker!" in the overhead shot, that side of the doors (in the hallway) is once again visible, only they look completely different now. (01:47:25 - 01:49:15)
Continuity mistake: Just as Mikaela comes to the rescue with the reciprocating saw, when she holds the saw up in front of her face just before she starts to hack away at Frenzy, the saw's blade clamp assembly is about 8-9 inches. After Sam kicks Frenzy, the saw on Mikaela's shoulder has a different configuration with a shorter blade clamp assembly unit. The blade differs as well. (00:54:25)
Continuity mistake: When we hear the Qatar base commander say: "4500X, something's not right," we see the sun has already set (the sky is purple) but when everyone starts to mobilize, the sun is still visible above the horizon.
Continuity mistake: When Bumblebee is caught and then frozen, you can tell that he isn't animated and is rather an enormous prop, because he is a lot cleaner than his animated counter-part.
Continuity mistake: In the scene in the interrogation room with the two programmers there is a big plate of doughnuts which the hacker eats very quickly. But take a look at the plate between shots when he's talking and you'll see that the amount of doughnut left on the plate varies greatly: Sometimes there's one whole doughnut left, other times, in the same conversation, there's only crumbs.
Continuity mistake: When Captain Lennox's wife is watching the broadcast saying that everyone at Qatar was killed, the door is wide open. When she sits down to feed her baby, the door is now closed and the high chair has been moved accordingly.
Continuity mistake: In the scene when Sam is being chased by two dogs and is led into a strange-looking place, when Sam climbs on top of a barrel, notice that the dogs disappear once Bumblebee drives in. Also notice that when Bumblebee drives into that place, he does a 180-degree slide, and in the next shot he's seen circling around Sam. (00:34:10)
Continuity mistake: When the news crew is interviewing Sam's parents during the credits, the way Ron holds Mojo changes. (02:15:10)
Continuity mistake: After Sam and Mikaela arrive at the Griffith Park Observatory (the planetarium closed for renovation), when they start to hold hands the distance between the two of them changes from farther apart, to close together, to farther apart. (00:59:35)
Continuity mistake: When Banacheck shows Sen. Kelly comparative photos of Blackout with the one from the beagle probe and Epp's photo, the latter one shows the sun behind Blackout although it was taken at night.
Continuity mistake: During the chase scene, where Bonecrusher and Barricade are chasing the Autobots on the highway, Bonecrusher transforms into robot mode and is shown skating across a 6 lane highway. Optimus Prime is shown to transform into his robot form to fight Bonecrusher. You can see the 6 lane highway in front of Prime stretch on as far as the eye can see, and there are no ramps or overpasses anywhere in sight. Yet at the moment of impact, when Bonecrusher tackles Prime, they are suddenly on a highway overpass, which they fall off of, and fall onto ANOTHER highway overpass.
Continuity mistake: Mikaela begins cuffing the undershirt/boxer wearing Agent Simmons arms around a post in the wideshot, then it cuts to the close-up, as she handcuffs the dark suit wearing right wrist of one agent to the dark suit wearing left wrist of another agent. Then in the two following shots we see that she and Sam have actually just cuffed the undressed Agent Simmons left wrist to the dark suit wearing second agent. (01:24:00)
Continuity mistake: After Mikaela and Sam walk away from all the handcuffed agents, when we hear an Autobots shout, "Optimus! Incoming!" the railroad crossing post is visible behind the Autobots, but Agent Simmons is not handcuffed to it. Then, when we hear, "Roll out!" Agent Simmons is handcuffed to that post as he should be. (01:24:20)
Continuity mistake: When Mikaela is telling Captain Lennox that she'll leave as soon as she gets Bumblebee out of the area, watch her hair. At first, her back is to the camera and her hair is perfectly smooth but when the camera is facing her, her hair is extremely messy. (01:57:55)
Continuity mistake: At the beginning of the movie, during Blackout's inbound flight, Blackout is ten miles out. When the two F-22s take off, the angle between the planes and their shadows on the ground is around 60°. This puts the sun somewhere in the middle of the day. Yet, when Blackout lands at the base it's just after sunset.
Continuity mistake: When the Autobots arrive at Sam's house, we only see Optimus, Jazz, and Bumblebee driving up to the house. But when they transform, Ironhide and Ratchet have appeared and Jazz is nowhere to be seen. Later, Jazz appears from nowhere. Also, when Sector 7 arrives, the Autobots are gone without a trace. (01:09:20 - 01:15:50)
Continuity mistake: When Sam and his friend drive into the park to attend the party, the colour and angle of the sun's rays seem to indicate that the sun will be setting soon. However, when Sam starts talking to Michaela's boyfriend, the sun is much higher and brighter than when Sam arrived a few minutes earlier.
Continuity mistake: When Sam is looking at Bumblebee at the car lot, he wipes some of the grease off of the Autobot insignia. When Bumblebee blows out all the windows, all the grease is gone. (00:13:55 - 00:14:50)
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.
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