A pathological SVG file can do this:

<svg>
  <defs>
    <rect id="foo" .../>

    <g id="foo1">
      <use xlink:href="#foo"/>
      ... repeat 10 times ...
    </g>

    <g id="foo2">
        <use xlink:href="#foo1"/>
        ... repeat 10 times ...
    </g>

    <g id="foo3">
        <use xlink:href="#foo2"/>
        ... repeat 10 times ...
    </g>

    ... etc ...
  </defs>

  <use xlink:href="#foo17"/>
</svg>

This would cause about 10^17 objects to be rendered.  While this does
not exhaust memory (the objects are not instanced in memory), it would
take a really long time to render that many objects.

So, we now have a limit on up to 500,000 objects instanced through
<use>.  We can tweak this limit later, or the way in which it is
computed; the point is that we can now detect this situation and
propagate an error upstream.

#323