Alrighty, here is the best answer based on the issue I encountered in Magento 1.9.3.8 after installing it fresh. Replace the vsprint line in the app\code\core\Mage\Core\Model\Translate.php:416 with:

try {
    $result = vsprintf($translated, $args);
} catch (Exception $e){
    echo $translated;
    exit;
}

This will print the issue string on the screen. For me it was this one: Please input a valid CSS-length. For example 100px or 77pt or 20em or .5ex or 50%. Then you can search the code for the string and escape it if it's your code. If not, then you found a bug in someone else's code. Submit a bug report to them so they can fix it. In this case, this is in Mage_Core, so it's a Magento bug that was overlooked when releasing.

Alrighty, here is the best answer based on the issue I encountered in Magento 1.9.3.8 after installing it fresh. Replace the vsprint line in the app\code\core\Mage\Core\Model\Translate.php:416 with:

try {
    $result = vsprintf($translated, $args);
} catch (Exception $e){
    echo $translated;
    exit;
}

This will print the issue string on the screen. For me it was this one: Please input a valid CSS-length. For example 100px or 77pt or 20em or .5ex or 50%. 
Then you can search the code for the string and escape it if it's your code. If not, then you found a bug in someone else's code. Submit a bug report to them so they can fix it. In this case, this is in Mage_Core, so it's a Magento bug that was overlooked when releasing.