title: Rails (rarely) attempting to load old pack files
body: <p>I am using new relic with my app, and it seems there are often 404s that show up.  The file it is trying to load is an older <code>packs/js/application-blah.js</code>, sometimes one revision old, sometimes the very first revision (10+ deploys and weeks ago).  This started happening (I think) when upgrading to webpacker 4 from 3.5.</p>
  
  <p>Over a 12 hour period I saw 10 errors for the 404, and served an average of 40 pages per minute during the time frame, so it is decently rare and I can't reproduce it.  It does not seem to be OS or browser specific, I'm seeing it on everything from Edge on Windows to Safari on iOS 12.</p>
  
  <p>Also seems to be trying to bring in a file from <code>packs/</code>, which I don't think should exist anymore with webpacker 4?</p>
  
  <p>This is an excerpt what I see in New Relic:</p>
  
  <pre><code>No route matches [GET] "/packs/js/application-1a5742633d44555a15cf.js"
  No route matches [GET] "/packs/application-b8e132938408ef6c4ee9.js"
  </code></pre>
  
  <p>This is what <code>packs/manifest.json</code> looks like:</p>
  
  <pre><code>{
   "application.js": "/packs/js/application-63945be2fec97ef46418.js",
   "application.js.map": "/packs/js/application-63945be2fec97ef46418.js.map",
   "entrypoints": {
     "application": {
       "js": [
         "/packs/js/application-63945be2fec97ef46418.js"
       ],
       "js.map": [
         "/packs/js/application-63945be2fec97ef46418.js.map"
       ]
     }
   }
  }
  </code></pre>
  
  <p>The errors show up for a day or two after a deploy, but did not show up this past weekend (no deploys on Friday), so it makes me think it is a caching issue.  The <code>b8e...</code> it is trying to load has always been the same though, that never changes.</p>
  
  <p>I ran a script to download the page and look at the head to see what it is trying to serve, and only ever see the correct file, so I suspect it might somehow be client side related?  The app uses turbolinks, but if it is trying to grab pack files then that should mean its a full page load, so turbolinks should not have any old files cached.</p>
  
  <p>If anyone has any insight to offer I would be greatly appreciative!</p>