With markup:

<table id="mytable">
    <tbody>
        <tr class="one">
            <td></td>
        </tr>
        <tr class="two">
            <td></td>
        </tr>
    </tbody>
</table>

This works properly:

$("#mytable tbody > tr:last-of-type");  // [ tr.two ]

But this will return all the tr nodes:

$("tbody >tr:last-of-type", "#mytable"); // [ tr.one, tr.two ]

So will this:

$("#mytable tbody").children("tr:last-of-type"); // [ tr.one, tr.two ]

:last-child works in all of these cases.

:first-of-type exhibits the exact same broken behaviour.