mitmproxy/doc-src/faq.html

<div class="faq">

    <p class="question">On some sites I see a lot of "Connection from.."
    entries that never complete.</p>

    <p> This is probably because the page requests resources from SSL-protected
    domains. These requests are intercepted by mitmproxy, but because we're
    using a bogus certificate, the browser-side of the connection hangs. The
    browser doesn't prompt you to add a certificate trust exception for remote
    page components, only for the primary domain being visited. </p>
    
    <p> To solve this, use something like FireBug to find out which page
    components are hanging. Visit the relevant domains using your browser, and
    add a certificate trust exception for each one. </p>

</div>