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Be careful if you use Mage_Core_Block_Abstract::toHtml() inside an observer for core_block_to_html_after. I did that to inject HTML from another block into an existing block with the result that everything except this new block was removed from output.

Digging through the core, I found the cause in how the event is dispatched:

        /**
         * Use single transport object instance for all blocks
         */
        if (self::$_transportObject === null) {
            self::$_transportObject = new Varien_Object;
        }
        self::$_transportObject->setHtml($html);
        Mage::dispatchEvent('core_block_abstract_to_html_after',
            array('block' => $this, 'transport' => self::$_transportObject));
        $html = self::$_transportObject->getHtml();

As you see, the transport object that is used to allow modification of the HTML in observers, is a class variable of Mage_Core_Block_Abstract, so basically acts like a singleton. I cannot think of a good reason for this, but it smells ridiculously like premature optimization.

Now what’s happening is, that during the event is processed, the new block sets its own output to the same transport object in its toHtml() method and the original output is lost, if you didn’t save it before.

The solution to my particular problem was to generate and pre-render the new block elsewhere but if you don’t, you need to copy the output at the very beginning:

$html = $observer->getTransport()->getHtml(); // should be the first statement

// now you can do what you want, create other blocks, modifiy $html ...

$observer->getTransport()->setHtml($html); // should be the last statement