Helper can not be mocked

[ScreamingDev]

Because the correct method wasn't used.

[IvanChepurnyi]

Which helpers do you mean? It works great for specifying full class alias of helper.

[ScreamingDev]

I tried to mock my own Helper/Data.php and the returned class were named "...LeMike_DevMode_Helper_012456" which is obviously wrong.

Even $this->replaceByMock('helper', 'core', $helperCoreMock); and I had to write 'core/data' instead which is not very common if you think of Mage::helper('core') for example.

getHelperClassName is the only method in magento that resolves the name for helper correct.

[IvanChepurnyi]

$this->replaceByMock() should not store value in registry with /data in the end, if version of Magento is greater than CE 1.6. Otherwise it won't be mocked. If you check the core code, you will get the point.

    public static function helper($name)
    {
        $registryKey = '_helper/' . $name;
        if (!self::registry($registryKey)) {
            $helperClass = self::getConfig()->getHelperClassName($name);
            self::register($registryKey, new $helperClass);
        }
        return self::registry($registryKey);
    }

As for getHelperMock, I will merge the change.