If you have a block structure that is something similar to the following:

<container>
    <item style="margin-bottom: 10px"></item>
    <item style="margin-bottom: 10px; height: 0px;">
        <item style="margin-bottom: 10px; height: 0px;">
            <item style="margin-bottom: 10px; height: 0px;">
                <item style="margin-bottom: 10px; height: 0px;"></item>
                <item style="margin-bottom: 10px; height: 0px;"></item>
            </item>
        </item>
    </item>
</container>

We currently trim the last child, which is self collapsing, and its margins correctly, but none of the children nested within it are trimmed. Since these are at the block-end and we trim  the margins that are collapsed through for the container, none of the inner margins end up participating in margin collapsing with the container's margin and renderings look correct. However, with the tests added as a part of 253610 we can actually see that these margins are not trimmed by looking at the computed style values.