When you forget to extend `React.Component` you get the following error: ``` TypeError: Cannot call a class as a function at _classCallCheck (null.js:7:99) at Hello (null.js:11:5) ...react internals... ``` [[Example](http://react.jsbin.com/?html,js,console)] It would be nice if in dev you could do a check for `Component.prototype.render` and if it exists, provide a better warning along the lines of: ``` Warning: Using a class with a render method as a function, did you forget to extend React.Component? ``` I believe it would happen in [ReactCompositeComponent.js](https://github.com/facebook/react/blob/b840229286ac2a82fa49553ce793cf7b953d1845/src/renderers/shared/stack/reconciler/ReactCompositeComponent.js#L248).