lib/Twig/lib/Twig/TokenParser/For.php
protected function checkLoopUsageBody(Twig_TokenStream $stream, Twig_NodeInterface $node)
    {
        if ($node instanceof Twig_Node_Expression_GetAttr && $node->getNode('node') instanceof Twig_Node_Expression_Name && 'loop' == $node->getNode('node')->getAttribute('name')) {
            $attribute = $node->getNode('attribute');
            if ($attribute instanceof Twig_Node_Expression_Constant && in_array($attribute->getAttribute('value'), array('length', 'revindex0', 'revindex', 'last'))) {
                throw new Twig_Error_Syntax(sprintf('The "loop.%s" variable is not defined when looping with a condition.', $attribute->getAttribute('value')), $node->getTemplateLine(), $stream->getSourceContext());
            }
        }

        // should check for parent.loop.XXX usage
        if ($node instanceof Twig_Node_For) {
            return;
        }

        foreach ($node as $n) {
            if (!$n) {
                continue;
            }

            $this->checkLoopUsageBody($stream, $n);
        }
    }
protected function checkLoopUsageBody(Twig_TokenStream $stream, Twig_NodeInterface $node)
    {
        if ($node instanceof Twig_Node_Expression_GetAttr && $node->getNode('node') instanceof Twig_Node_Expression_Name && 'loop' == $node->getNode('node')->getAttribute('name')) {
            $attribute = $node->getNode('attribute');
            if ($attribute instanceof Twig_Node_Expression_Constant && in_array($attribute->getAttribute('value'), array('length', 'revindex0', 'revindex', 'last'))) {
                throw new Twig_Error_Syntax(sprintf('The "loop.%s" variable is not defined when looping with a condition.', $attribute->getAttribute('value')), $node->getTemplateLine(), $stream->getSourceContext());
            }
        }

        // should check for parent.loop.XXX usage
        if ($node instanceof Twig_Node_For) {
            return;
        }

        foreach ($node as $n) {
            if (!$n) {
                continue;
            }

            $this->checkLoopUsageBody($stream, $n);
        }
    }
it does not catch all problems (for instance when a for is included into another or when the variable is used in an include)