title: 'Failed to process features.' error from Google Vision API
body: <p>Sometimes <a href="https://vision.googleapis.com/v1/images:annotate" rel="nofollow noreferrer">https://vision.googleapis.com/v1/images:annotate</a> request returns strange error: "Failed to process features." What does it mean? The issue is very rare. The most requests are succeeded.</p>
  
  <p>I did not find any specific in failed requests. I tried to reproduce the issue by sending incorrect requests (bad type instead of "DOCUMENT_TEXT_DETECTION", empty "features" section and etc.) But error message is not "Failed to process features." in that cases.</p>
  
  <p>Request body:</p>
  
  <pre><code>  {
      requests: [
        {
          image: {
            content: &lt;base64 encoded PNG file&gt;
          },
          features: [
            { type: "DOCUMENT_TEXT_DETECTION" }
          ]
        }
      ]
    }
  </code></pre>
  
  <p>Image is small: 480x18px, 3514 bytes. Request execution time is normal (~300 ms).</p>