This usually means that the user mixed up our lowercase/uppercase naming convention; see https://github.com/facebook/react/pull/3429#issuecomment-85166440.

We could add a message like

> Warning: The tag <commentBox> is unrecognized in this browser. If you meant to render a React component, start its name with an uppercase letter.

We'd check this only once per tag and only in development mode.

render() {
          return <div><Child1 /><Child2 /><Child3 /><Child4 /></div>;
        }
      }
      class Child1 extends React.Component {
        render() {
          return <div class="paladin">Child1</div>;
        }
      }
      class Child2 extends React.Component {
        render() {
          return <div>Child2</div>;
        }
      }
      class Child3 extends React.Component {
        render() {
          return <div onclick="1">Child3</div>;
        }
      }
      class Child4 extends React.Component {
        render() {
          return <div>Child4</div>;
        }
      }
      ReactDOMServer.renderToString(<Parent />, container);
      expectDev(console.error.calls.count()).toBe(2);
      expectDev(console.error.calls.argsFor(0)[0]).toContain('className');
      var matches = console.error.calls.argsFor(0)[0].match(/.*\(.*:(\d+)\).*/);
      var previousLine = matches[1];
      expectDev(console.error.calls.argsFor(1)[0]).toContain('onClick');