When you edit a richtext, you can switch to the "html code mode".
If you type some invalid code, it is highlighted in red.
If you then switch back to the normal view mode, the code is fixed (as best as it can)
If, instead, you try to save the content, the invalid HTML code is send (but it should not be).

You get the following error on save :

Impossible de transformer le texte riche : Exception during processing of cocoon://_plugins/cms/convert/html2docbook.