Howto recover from a broken onbeforeview template
Up to and including Ariadne 2.7.6 the onbeforeview event is broken. If you add a listener for this event in a config.ini template — which is the only place it makes sense — it will break Ariadne and you cannot then fix the config.ini template anymore.
The solution is to apply this patch to lib/objects/ariadne_object.php. This will allow you to use the onbeforeview event as intended. You can still very easily break your website by making a programming error in your event handler, but this is now easily fixed by editing your template and correcting the error.