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In general, the way the Bay films each try to outdo their last entry in terms of secret conspiracies and convoluted mcguffin plots, can end up being this. At least first time around it wasn't too terrible besides the cliché nature of the Allspark's importance, but as the sequels got on it just became ridiculous. The way they'd somehow be able to stumble upon another, unrelatedAncient Conspiracy involving the Transformers in human history that would add more convoluted layers to their mythology, ultimately just turning it into an unintended joke.
Depending on who you ask, Bumblebee's mouth looks like a gag or a pacifier.
The opening monologue. Optimus Prime's wise, deep voice says, "Before time began, there was... THE CUBE."
"The Allspark is two hundred and thirty miles from HERE!"
Most notable is Sam's hilariously over-the-top, melodramatic reaction to finding out that Mikaela has a criminal record. Not only is this not something that's a huge deal in and of itself, but all of the Autobot-military stuff going on around him right then makes it look unimportant by comparison.
Bonecrusher's Angrish comes off as awesome and terrifying, except when he transforms, for which you will find many people who think that he says "WANT TEA".
During the otherwise spectacularly rendered forest battle, there's a very glaring animation shortcut, where, for a split second, Megatron goes from charging furiously at Optimus to strolling complacently. It's partially obscured by the nature of the shot, but many sharp-eyed viewers found it hilarious.
Megatron dramatically yells "All. Hail. Megatron!". At a bunch of elephants and zebras. The implication there seemed to be that Megs was a tad delusional by that point, given his horrific head injury and all. Still, at least it wasn't as bad as it was in the novel and comic, wherein he follows up said yell by randomly blowing up an elephant with his fusion cannon.
If not that, his telling off Dylan Gould certainly qualifies:
Optimus' strange sputtering as the Driller sends him flying.
The RUNFAAAGH! used by Sentinel towards the end of the final battle. The fact that's it's a stock sound effect from the previous film, from Megatron no less, doesn't help.
Sam's complete meltdown and utterly ridiculous screaming when the security tries to impound his car.
Starscream's utterly pathetic death. Even disregarding the silliness of Sam of all people being able to take out the second-in-command of the Decepticons, Starscream's behavior during the scene (flailing about wildly, spitting out oil(?) whenever he opens his mouth) make it impossible to take seriously. Then again, given that it's Starscream, this may be the point.