This is harsh.
As if Pixar movies weren't already brutal enough, these kids took things a step further by editing Toy Story 3 so that the credits start to roll right in the middle of the incinerator scene. You know the one - where Woody has given up his struggle to escape, and all the toys have joined hands as they descend to their certain death:
In the movie, the moment is pretty grim - the toys have accepted that they're going to die and are have given up any hope of living. It really seems like there's no way out - until, of course, the Pizza Overlord aliens grab them at the last moment with a crane, which leads to the (still sad) happier ending of Andy giving away his toys to a new kid who would be able to play with them.
And that's why this prank is so unapologetically brutal - some kids edited Toy Story 3 in Final Cut so that the incinerator scene is the actual finale of the movie, cutting to credits RIGHT as all the toys were seemingly about to die. They showed their version of the film to their (rightfully) horrified mother...
...and then let her think that was the ACTUAL ENDING for at least a full day.
It's brutal. It's cruel. It's the perfect prank.