This tests the clone limit; you can go in the code and change what scripts are active to test the limit in different ways.
Ok. I got help from @DadOfMrLog and he told me that the clone limit is, in fact, 301. He even showed me the lines in the Scratch2 source code that set this limit. I'm a bit salty about that. …But then Scratch 3.0 rolled along and it looks like they set the limit to 300! Hooray!