Access invalid or inaccessible notebook ID (e.g. https://observablehq.com/d/eb0f603814a2719c)
Page shows 404 Not Found
Page stays blank, shows banner “Welcome. This is live code! Click the left margin to view or edit.”.
An initial test actually resulted in an exception loop that spammed analytics and resulted in 100% CPU usage. I wasn’t able to reproduce this case, though.