So, I'm facing the almighty "Class Bla not found" error on one of my website.

The class file sits in the vendor folder and I'm using an event observer for the autoload:

<events>
    <resource_get_tablename>
        <observers>
            <composer_autoloader>
                <type>singleton</type>
                <class>module/observer</class>
                <method>addComposerAutoloader</method>
            </composer_autoloader>
        </observers>
    </resource_get_tablename>
</events>

Here's my observer:

class Vendor_Module_Model_Observer
{
    /**
     * @var bool
     */
    protected static $added = false;
    /**
     * Register the Composer autoloader
     * @param Varien_Event_Observer $observer
     */
    public function addComposerAutoloader(Varien_Event_Observer $observer)
    {
        if (self::$added === false) {
            /** @var $helper Vendor_Module_Helper_Data */
            $helper = Mage::helper('module');
            $helper->registerAutoloader();
            self::$added = true;
        }
    }
}

And here's my helper method:

public function registerAutoloader($path = null, $filename = null)
{
    if ($path === null) {
        $path = $this->getVendorDirectoryPath();
    }
    if ($filename === null) {
        $filename = self::AUTOLOAD_FILENAME;
    }
    if (file_exists($path . DS . $filename)) {
        require_once($path . DS . $filename);
    }
}

According to my tests using xDebug, the observer is properly triggered and the require_once leading to my vendor/autoload.php is called.

I've checked the namespace I use to call my custom class and I'm 100% sure there's no typo.

NB: please note that if I get rid of this composer code and if I patch the app/Mage.php directly with the following code, it does not work either:

/** AUTOLOADER PATCH **/
if (file_exists($autoloaderPath = BP . DS . '../vendor/autoload.php') ||
    file_exists($autoloaderPath = BP . DS . 'vendor/autoload.php')
) {
    require $autoloaderPath;
}
/** AUTOLOADER PATCH **/

So my question is: how do I debug that error ? Any usual suspects ?