The following test fails:

class MyTest extends Zend_Test_PHPUnit_ControllerTestCase
{
    public function testFoo()
    {
        $this->dispatch('/');
        $this->assertQuery('form#foo');
    }
}

when requested document begins with XML declaration:

<?xml version="1.0" encoding="UTF-8" ?>

and passes if there is no such declaration.

What is more interesing

$this->assertQuery('#foo');

passes. Same applies to corresponding XPath queries (CSS queries are translated into XPath queries):

$this->assertXpath('//form[@id="foo"]');

fails, but the following passes

$this->assertXpath('//*[@id="foo"]');