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.
knownKeys = warnOnInvalidKey(child, knownKeys);
        }
      }
    }
    const newChildren = iteratorFn.call(newChildrenIterable);
    invariant(newChildren != null, 'An iterable object provided no iterator.');
    let resultingFirstChild: Fiber | null = null;
    let previousNewFiber: Fiber | null = null;
    let oldFiber = currentFirstChild;
    let lastPlacedIndex = 0;
    let newIdx = 0;
    let nextOldFiber = null;
    let step = newChildren.next();
    for (
      ;
      oldFiber !== null && !step.done;
      newIdx++, (step = newChildren.next())
    ) {
      if (oldFiber.index > newIdx) {
        nextOldFiber = oldFiber;
        oldFiber = null;
      } else {
        nextOldFiber = oldFiber.sibling;
      }
      const newFiber = updateSlot(returnFiber, oldFiber, step.value, priority);
      if (newFiber === null) {
        // 
        // 
        // 
        //