Go to the stage.
Then grab a switch to costume block.
Then, grab a stretch costume current block and put it in your switch to costume block.
If you run it, it will black out the website and the only way to stop it is by reloading the website. Please patch this. 
Funny!
and i know your not like:
It's not a browser bug (tested in chrome and firefox)
It append only if the background is empty
Phase1: the stage become black (notice the block still running)
Phase2: when you clign on the scripting area:
I don't think strech costume is meant for the stage.
The problem is not the stretch block itself, it's setting the stage costume to a costume that is not the size of the stage.
What do you mean, why was I replying? Can't I help people out?
and if you do it twice, there will be an error message
Empty@Stage is a special form (Turtle equivalent) and has no pixel data.