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Nightmare Fuel: MythBusters
The Meatman is pretty disturbing in itself, but the test it's used for— showing that a person could be crushed in a closed-dress diving suit by deep water pressure— will haunt you for a good while. Watch... if you dare.
Of course it also quickly becomes an understated Moment Of Awesome for Kari, because she stops him. She takes a moment to calm herself down, then continues with the experiment until the attending EMT pulls rank. Kari Byron, Woman Of Iron.
The high-speed camera also counts, particularly when showing anything biological.
There are two words guaranteed to make long-time fans cringe: "Chicken Hand".
Also smashing quick-frozen heads to see if they would shatter like glass (as shown in Jason X). They brought out their trusty ballistics gel again, used a similar substance for brains...and used actual human skulls. Then for even more added realism, they used pigs' heads.
They even said in the show that they couldn't show the original clip just because it was so disturbing, and had to substitute with a cheesy cartoon re-enactment.
Adam later stated in a web video that this was the most dangerous stunt he'd ever done, as they try not to put themselves in situations where not panicking can make the difference between life and death.
Likewise, Jamie's time being buried alive...and the coffin started buckling.
As stated in the "A-Z of Explosions" special, nothing. As in, when they try to blow something up and nothing happens, because this means someone has to go over and find out what went wrong and fix it, putting themselves in the potential blast radius of armed explosives.
Though it was never actually aired, during filming of the Build Team's test of decapitating heads (using goats' heads as stand-ins), Tory mentioned he found how easy decapitation is particularly disturbing.