Revealing mistake: When Blackout is destroying the base in Qatar, a few Humvees get flipped over, and none of them have engines in them. (00:07:00)
Revealing mistake: When Mikaela and Sam are in his car when he gives her a ride from the lake, both windows are open but her hair doesn't blow around at all - like it would if they were really open. (00:23:00)
Revealing mistake: At night, when Sam's car is running away by itself, you can see out of the windows that it was filmed in broad daylight. (00:33:45)
Revealing mistake: When Sam flips over on the bike, in one shot you can see that it is a stuntman who looks much older than Sam. (00:47:55)
Revealing mistake: When Sam Witwicky's great-grandfather activates Megatron's navigation system, he is blown back. As he falls, you can see one of Megatron's robotic fingers wiggle. (01:06:35)
Revealing mistake: When Devastator (the tank-bot) is activated and exits its base through a fence, the upper, obviously-rubber, cannon barrels wiggle around. (01:39:15)
Revealing mistake: When Bonecrusher throws the car off the car-carrier, you can see that the underside is just a flat black-painted surface with no gas tank. (01:49:55)
Revealing mistake: When the bus splits after Bonecrusher smashes through it, you can see that it has no seats or passengers. (01:50:35)
Revealing mistake: When Ironhide shouts, "Incoming!" as Starscream fires on them, Lennox and Epps are blown backwards and slam onto the ground. The shot of them hitting the ground and the next close-up of Lennox are flipped shots. Note the reversed 'Broadway Bar' sign and Lennox's watch. (01:53:45)
Revealing mistake: A yellow Pontiac GTO gets shot up, then gets blown in half with the rear end landing inside a shop window. If you look at the sideskirt of the car before it gets destroyed, you can see the cut already in the sideskirt where it will split apart. (02:01:20)
Revealing mistake: In the final battle, Captain Lennox is going to shoot Blackout. He gets on a motorcycle, goes really fast, and then slides off the bike and down the street. While he is sliding, he shoots Blackout. When Lennox gets to his feet, you can see his back, and his shirt is not worn away at all. It isn't even discolored.
Revealing mistake: When Blackout sees the green laser light on him, he turns around and flips over some cars, a couple of which don't have engines in them.
Revealing mistake: Near the beginning Shia and Megan Fox are driving in Bumblebee, before it breaks down. There is no road shown in this entire scene, no matter which angle you see. No landscape either. This indicates the scene was probably shot on a soundstage and the car was not moving. It is also obvious there is no wind coming in through the windows.
Revealing mistake: When Optimus Prime is powersliding into the city, right before he transforms, you can see that the truck is actually a CGI replica, because the blue is much lighter than it was a second ago, when he already burst out of the shadow of the alley.
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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