Take a look at this project: http://beta.scratch.mit.edu/projects/10044072/

The Scratch team should look at this as an example of how someone can inadvertently lock up the browser by creating lots of clones in a tight loop. It seems like there should be something that guards against this.

Browser: Firefox 18.0.2

Woah, crashed mine! But instead of the browser flash. It ALMOST crashed my browser, and i am using SRWare Iron (google chrome that doesn't tack as many cookies)
EDIT: You can even easily make it so that if the user goes into the scripts, it will crash the browser!