Other mistake: When Frenzy attacks Sam, the teenager wears straight leg jeans with a narrow leg opening (not boot cut) and thick-soled Nikes, seen in numerous shots, as he kicks at the Decepticon. After the cut to Mikaela (who finds the reciprocating saw), during which Frenzy has already pulled off the bottom of Sam's jeans, Sam continues to kick and scream just before he gets up and runs. Problem is Sam still wears both thick-soled Nikes, neither of which slipped off as Frenzy maniacally pulled the jeans, with narrow leg openings, from his kicking legs/feet. Impossible even in the best of circumstances.
Other mistake: During the chase scene on the highway, when Sam says, "It's the same cop," they pan to a wide shot to show the whole freeway with the cars on it. All of the vehicles are on the wrong side of the freeway. When looking out of the back of his car, you can see the fronts of all of the cars coming towards the camera, and you can see the text on the signs facing the camera as well. You can also see crowds on the flyover, watching the filming. (01:50:05)
Other mistake: When Sam and Mikaela are parked at the top of a cliff after "breaking down", Mikaela lifts the Bonnet of Bumblebee to check the engine. Mikaela, who's the car expert, states the motor has a carburetor, but the manifold on the engine was one for an individual throttle body fuel injection system. (00:24:50)
Other mistake: During the last fight, Megatron and Optimus Prime fly through an office building while locked in combat. It is shown that the interior of this building is filled with both people and furniture, but when they crash through the last wall, only building debris fall to the ground with them. No people, furniture, office supplies, papers or any of the things that the building was full of.
Other mistake: When Optimus Prime comes powersliding into Mission City, take a close look at the ground. You can see skidmarks from previous attempts. (02:00:00)
Other mistake: When Optimus picks up the glasses from the edge of the river, his hands are really out of size: the glasses are nearly as big as his thumb, while minutes before, they were big enough to hold two teenagers. (01:29:15)
Other mistake: When checking the eBay auction for his ancestor's glasses, the seller name is listed as 'Ron Witwicky'. eBay would list Sam's user name - 'ladiesman217' - as the seller, not a full name that isn't even his. It is clearly established throughout the film as the glasses are being sold through Sam's own account. (00:17:40)
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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