I made an example here: <http://jsbin.com/divubozaha/edit?js,console,output>

Basically, this has no warning:

```
<div>
  <h1>test1</h1>
  {Math.random() > 0.5 ? <h1>test1</h1> : null}
  <h1>test1</h1>
</div>

```

(even when some of the child can be re-ordered.

But this triggers the infamous `Each child in an array or iterator should have a unique "key" prop.`:

```
<div>{[
  <h1>test2</h1>,
  <h1>test2</h1>,
  <h1>test2</h1>
]}</div>

```

So, when filtering the children of an element, I have the warning. Maybe there's some kind of implicit keys in the fragments ?