While running a debug instance of Chromium, the following NOTREACHED() check fired, in ServiceWorkerProcessManager::RemoveProcessReferenceFromPattern():

  PatternProcessRefMap::iterator it = pattern_processes_.find(pattern);
  if (it == pattern_processes_.end()) {
    NOTREACHED() << "process references not found for pattern: " << pattern;
    return;
  }

The call stack didn't have much context: it was a posted task, the TaskAnnotator indicated that ServiceWorkerProcessManager::RemoveProcessReferenceFromPattern() had been PostTasked from an invocation of RemoveProcessReferenceFromPattern() on a different thread.

|pattern| was "https://www.facebook.com/", and I believe I was on the Google search result page for the query 'facebook'.

I've gotten it to happen a second time (though not consistently) via the following steps:
 - Open a new tab
 - Type 'facebook' into the omnibox (don't hit enter yet)
 - Using the arrow keys, select the omnibox dropdown option corresponding to the Google query for 'facebook' (not navigation to facebook.com)
 - Hit enter.

This definitely looks like a race of some sort.