jQuery.map(....).concat is not a function
<p>Why would the following jquery code sometimes throw the error "concat is not a function":</p>

<pre><code>var myArray = $('div.foo')
    .filter(function() { return $(this).is('.something'); })
    .map(function() { 
        return [['a', 'b', $(this).val()]];
    });

return myArray.concat(anotherArray);
</code></pre>
