title: What makes React.memo unreliable for preventing renders?
body: <p>In the <a href="https://reactjs.org/docs/react-api.html#reactmemo" rel="noreferrer">documentation for React.memo</a>, it says:</p>
  
  <blockquote>
    <p>This method only exists as a performance optimization. Do not rely on it to “prevent” a render, as this can lead to bugs.</p>
  </blockquote>
  
  <p>I don't really understand this warning. Why can it not be relied on? How is it an optimization if it doesn't necessarily prevent renders?</p>
  
  <p>I wish they had gone more into depth here, but since they didn't, can someone elaborate on this? As is, I feel reluctant to use it at all because this warning makes its behavior seem very unclear.</p>