Make a block with a WARP around its definition:
turn visible stepping on, drag the slider to the left to make it single stepping, and click on this:
Next, change the definition of THING to this while the block is running:
The WARP you removed looks like this:
Next, press the step button, and Snap! crashes. It doesn't look like it does on my computer at least, but you can't do anything.
Here's the error message in the console:
Uncaught TypeError: Cannot read property 'world' of undefined at Process.errorBubble (threads.js:1216) at Process.throwError (threads.js:1177) at Process.expectReport (threads.js:1161) at Process.evaluateContext (threads.js:779) at Process.runStep (threads.js:693) at threads.js:367 at Array.forEach (<anonymous>) at ThreadManager.step (threads.js:365) at StageMorph.step (objects.js:8431) at StageMorph.Morph.stepFrame (morphic.js:3204)