https://jsfiddle.net/3apqnhmw/

```jsx
class Foo extends React.Component {
  render() {
     return <div>Foo</div>
  }
}

class App extends React.Component {
  render() {
    return Foo
  }
}
```

When `App` returns `Foo` instead of `<Foo />` accidentally, Fiber does not catch this.
);
      if (newFiber) {
        if (shouldTrackSideEffects) {
          if (newFiber.alternate !== null) {
            // 
            // 
            // 
            // 
            existingChildren.delete(
              newFiber.key === null ? newIdx : newFiber.key,
            );
          }
        }
        lastPlacedIndex = placeChild(newFiber, lastPlacedIndex, newIdx);
        if (previousNewFiber === null) {
          resultingFirstChild = newFiber;
        } else {
          previousNewFiber.sibling = newFiber;
        }
        previousNewFiber = newFiber;
      }
    }
    if (shouldTrackSideEffects) {
      // 
      // 
      existingChildren.forEach(child => deleteChild(returnFiber, child));
    }
    return resultingFirstChild;
  }
  function reconcileChildrenIterator(