Continuity mistake: When Bobby B. walks with Ron and Sam to the main used lot, where Bumblebee and the VW Beetle are located, most of the cars opposite Bumblebee and to its side actually change make, color, etc., from the time Ron makes the U-turn, to when all the windows shatter and shots between. Examples are: there is a camper parked in the fifth space which changes to a car; the yellow hatchback behind Sam when he opens the car door changes to a light blue car; and the two blue cars (one with a spoiler) parked beside Jaime, the ostrich, which change to a dark car and yellow hatchback. (00:12:10 - 00:15:10)
Continuity mistake: When Ron (holding a flashlight and lantern) is looking through Sam's bathroom window, saying, "Judy, better call the city. We got a blown transformer...," there is no windowbox under that window, which was there earlier. Yet when Prime says "Ironhide, you know we don't harm humans! What is with you?" the rather large windowbox full of flowers is back up under that bathroom window. (01:15:50)
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.
Continuity mistake: When one of the Autobots knocks out the power transformer at the Witwicky house the lights all go out, however in following shots the lights in the house have all come back on again.
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)
Continuity mistake: After Ratchet says: "The boy's pheromone levels suggest he wants to mate with the female", Mikaela scratches her head, but in the shot directly afterwards, her hand is instantly down. There is nowhere near enough time for her to put her hand down again. (01:03:50)
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)
Character mistake: Sam says that Archibald Witwicky explored the Arctic Circle in 1897. But when Frenzy finds the article, it is dated 1895.
Audio problem: When Lennox is speaking to his wife via the PC, when we hear him say, "Oh my goodness, look at her. She's getting so big," there is no mouth movement whatsoever. (00:05:05)
Continuity mistake: When Bumblebee's legs are destroyed, as Sam leans over in front of him and asks if he's alright, Sam wears blue patterned boxers, but when Lennox runs up to Sam, who is standing on the tow truck helping Mikaela hoist Bumblebee, Sam's boxers are grey (as seen between his white tee and jeans). Then, when Sam is on the roof running, after the chopper is hit, his boxers are blue patterned. (01:53:45 - 01:57:10)
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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