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)