Scratch

Here's a pretty simple example that will crash your browser (and, in theory, your computer):

Instant crash http://i61.tinypic.com/2vxhczl.png

I left this running for about twenty seconds, then lost 2.65 GB of memory to Scratch. Only a moment later and 5 GB were gone.

I highly suggest you have a means to force quit either Adobe Flash or your web browser before running this!

^{I really wanted to make a cool answer like the clear(this) JS one but sadly Scratch doesn't have any ways to do that. Feel free to update this post (or make your own) if you DO find another way to make Scratch unusable though!}

Scratch

Here's a pretty simple example that will crash your browser (and, in theory, your computer):

I left this running for about twenty seconds, then lost 2.65 GB of memory to Scratch. Only a moment later and 5 GB were gone.

I highly suggest you have a means to force quit either Adobe Flash or your web browser before running this!

^{I really wanted to make a cool answer like the clear(this) JS one but sadly Scratch doesn't have any ways to do that. Feel free to update this post (or make your own) if you DO find another way to make Scratch unusable though!}