Safe HTML / Très pratique si bloc bloquant !

This package provides a simple Safe template for the core HTML block. By assigning the Safe block template, the HTML is escaped in edit mode. So as long as you can get into edit mode, you will always be able to see where the broken block is and and edit it.

https://marketplace.concretecms.com/marketplace/addons/safe-html/

If you have trouble getting into the the page to edit it, you can hack a configuration value in the file 
application/config/generated_overrides/safe_html.php 
and set 'panic' => true. 
This will render the safe view no matter what, so you can edit the page and fix your faulty html. 
Remember to reset 'panic'=>false afterwards.