Turns out the CHECK I added for debugging can fail due to yet another bad side

chrome/browser/ui/omnibox/omnibox_navigation_observer.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/ui/omnibox/omnibox_navigation_observer.h"
 
 #include "chrome/browser/autocomplete/shortcuts_backend.h"
 #include "chrome/browser/autocomplete/shortcuts_backend_factory.h"
 #include "chrome/browser/infobars/infobar_service.h"
 #include "chrome/browser/intranet_redirect_detector.h"
@@ skipping 81 matching lines @@
   // system to synchronously navigate an extension background page.  Not only is
   // this navigation not the one we want to observe, the associated WebContents
   // is invisible and has no InfoBarService, so trying to show an infobar in it
   // later will crash.  Just ignore this navigation and keep listening.
   content::NavigationController* controller =
       content::Source<content::NavigationController>(source).ptr();
   content::WebContents* web_contents = controller->GetWebContents();
   if (!InfoBarService::FromWebContents(web_contents))
     return;
 
-  CHECK_EQ(match_.destination_url,
-           content::Details<content::NavigationEntry>(
-               details)->GetVirtualURL());
+  // Ignore navgiations to the wrong URL.
+  // This shouldn't actually happen, but right now it's possible because the
+  // prerenderer doesn't properly notify us when it swaps in a prerendered page.
+  // Plus, the swap-in can trigger instant to kick off a new background
+  // prerender, which we _do_ get notified about.  Once crbug.com/247848 is
+  // fixed, this conditional should be able to be replaced with a [D]CHECK;
+  // until then we ignore the incorrect navigation (and will be torn down
+  // without having received the correct notification).
+  if (match_.destination_url !=
+      content::Details<content::NavigationEntry>(details)->GetVirtualURL())
+    return;
+
   registrar_.Remove(this, content::NOTIFICATION_NAV_ENTRY_PENDING,
                     content::NotificationService::AllSources());
   if (fetcher_) {
     fetcher_->SetRequestContext(
         controller->GetBrowserContext()->GetRequestContext());
   }
   WebContentsObserver::Observe(web_contents);
   // DidStartNavigationToPendingEntry() will be called for this load as well.
 }
 
@@ skipping 49 matching lines @@
     OnAllLoadingFinished();  // deletes |this|!
 }
 
 void OmniboxNavigationObserver::OnAllLoadingFinished() {
   if (fetch_state_ == FETCH_SUCCEEDED) {
     AlternateNavInfoBarDelegate::Create(
         web_contents(), text_, alternate_nav_match_, match_.destination_url);
   }
   delete this;
 }