The cataclysm on Planet Zero. The four friends have to scale a rock wall to avoid the rapidly-approaching plasma and Victor loses his grip, with Reed's hand and a tether cord being the only things keeping him from falling. It looks like Reed might be able to pull him up, when a geyser of plasma splashes on Victor, burning his wrist, cutting through the tether and causing his helmet to melt onto his face (his agonized shrieking makes it even worse to watch). Reed's grip loosens when the plasma burns his hand and Victor falls into the newly-formed lake of plasma.
The aftermath of the protagonists' unauthorized trip to Planet Zero, especially for Reed. To the best of his knowledge, as a direct consequence of his own stupidity and recklessness, Victor has been dropped into an unidentified green energy and is either dead or stranded in a parallel universe with no food or water, a broken environment suit, and no way home and will most likely be dead within a week. Johnny fares little better; Reed can see what to anyone who doesn't know any better looks like him slowly and painfully burning to death. He can also hear Ben, covered in a ton of rocks, unable to move and begging him to help. When he tries desperately to crawl over to his childhood friend, he turns around... and realizes he's stretching.
The hallway attack scene where Doom head-explodes a number of people unfortunate enough to get in his way (and not even in his way, just kind of standing off to the side screaming in completely-justified horror) is genuinely unsettling, and surprisingly violent for a PG-13 film.
One of the bonus features reveals a rather unsettling detail about The Thing's condition. It turns out that Ben Grimm is still inside all of those rocks that make up his form.
