It seems like everything runs on open. So if you happen to copy paste some offending code, the browser just explodes when you enter the notebook.
How to cease execution on open?
Awesome! Yes that is exactly what I was looking for. Almost should be default or more obvious to fall into that I think. Very easy to write something that just clobbers that browser proc.
Also looks like as of time of writing you can just append /safe to your url to debug. Obviously api might change but this is good trick.