Even though Scratch made it so you can no longer install malware through games, I might have found a way. I can crash your page with only 2 lines of code and 5 blocks. How it works: When the green flag is clicked, the cat creates a clone of itself. Then, when it starts as a clone, it creates another clone and deletes itself; A loop that goes on forever. This creates a paradox so powerful it instantly crashes your Scratch page. It is also known by the government as a fork bomb, a way to install malware and make someones computer practically unusable. However, this fork bomb is much less powerful and doesn't destroy peoples computers. A normal fork bomb has a code process that splits in half into 2 more process's, which do the exact same thing forever until it breaks your computer. This is the same definition of what I have made. It will stop the game as fast as you clicked it, and then you can do other things on your device, but you can't press any button on your Scratch page. Wait for a few seconds, and then the page will crash. I accidentally made this while trying to make a forever loop without using a forever block. There are other similar ways to do this that don't crash your page, and they are so fast they will do anything instantly, making these forever loops even faster than the forever block.
Another thing you can do is make a project, add some code, then create a custom block called %15. It will not let you create the block, so press cancel, then see what you have done to Scratch. ST, please don"t ban me or take my game down for this, or fix the bug. I just made this for fun and at least I'm warning people about what it will do. Also, please don't report this project.