This code will not compile in the Dev. Console:
<!--docsample:nestedComponents-->
<aura:component>
    Observe!  Components within components!

    <docsample:helloHTML/>

    <docsample:helloAttributes whom="component composition"/>
</aura:component>


This code will compile:

<!--docsample:nestedComponents-->
<aura:component>
    Observe!  Components within components!

    <c:helloHTML/>

    <c:helloAttributes whom="component composition"/>
</aura:component>

The name space is wrong for the two nested components.  (Unless I am completely hallucinating!  And I might be - it's Friday!)