<h1 dir="auto">Bug report</h1>
<h2 dir="auto">Describe the bug</h2>
<p dir="auto">In some circumstances, <code class="notranslate">next build</code> is failing in CI environment (CircleCI) with the exception `Callback was already called'.</p>
<p dir="auto">There is a random factor and it's not happening 100% of the time, but more than 50% at the moment.</p>
<p dir="auto">This is triggered from the <code class="notranslate">neo-async</code> package which is used by <code class="notranslate">webpack</code>:</p>
<div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="/home/circleci/project/node_modules/neo-async/async.js:14
    throw new Error('Callback was already called.');
    ^

Error: Callback was already called.
    at throwError (/home/circleci/project/node_modules/neo-async/async.js:14:11)
    at /home/circleci/project/node_modules/neo-async/async.js:2805:7
    at afterBuild (/home/circleci/project/node_modules/webpack/lib/Compilation.js:859:17)
    at processTicksAndRejections (internal/process/next_tick.js:74:9)"><pre class="notranslate"><code class="notranslate">/home/circleci/project/node_modules/neo-async/async.js:14
    throw new Error('Callback was already called.');
    ^

Error: Callback was already called.
    at throwError (/home/circleci/project/node_modules/neo-async/async.js:14:11)
    at /home/circleci/project/node_modules/neo-async/async.js:2805:7
    at afterBuild (/home/circleci/project/node_modules/webpack/lib/Compilation.js:859:17)
    at processTicksAndRejections (internal/process/next_tick.js:74:9)
</code></pre></div>
<h2 dir="auto">Example</h2>
<p dir="auto">See for example: <a href="https://circleci.com/gh/opencollective/opencollective-frontend/5926" rel="nofollow">https://circleci.com/gh/opencollective/opencollective-frontend/5926</a></p>
<h2 dir="auto">Version</h2>
<p dir="auto">This is a new issue after upgrading to next 8.0.x, we never had it with next 7.0.x.</p>
<p dir="auto">If you look into the next.js doc you'll see:</p>
<p dir="auto"><a target="_blank" rel="noopener noreferrer nofollow" href="https://user-images.githubusercontent.com/15319886/31860423-b9ea81f4-b726-11e7-9c6f-fcb2651562f9.png"><img src="https://user-images.githubusercontent.com/15319886/31860423-b9ea81f4-b726-11e7-9c6f-fcb2651562f9.png" alt="image" style="max-width: 100%;"></a></p>
<p dir="auto">since I want to use this buildId to make a predefined service worker file(not using runtime caching), I wonder why buildId for app.js differs from other js files. you can see screenshot of this behaviour in my project:</p>
<p dir="auto"><a target="_blank" rel="noopener noreferrer nofollow" href="https://user-images.githubusercontent.com/15319886/31860444-089c1ab0-b727-11e7-89da-34a386290da6.png"><img src="https://user-images.githubusercontent.com/15319886/31860444-089c1ab0-b727-11e7-89da-34a386290da6.png" alt="image" style="max-width: 100%;"></a></p>
<ul dir="auto">
<li>[ x] I have searched the <a href="https://github.com/zeit/next.js/issues">issues</a> of this repository and believe that this is not a duplicate.</li>
</ul>
<h2 dir="auto">Expected Behavior</h2>
<p dir="auto">I think buildId should be a constant value between all files.</p>
<h2 dir="auto">Current Behavior</h2>
<p dir="auto">buildId for app.js is different from all other next.js generated js files.</p>