Trying to save a node form without the mercury editor enabled leads to a "Page not found" error and the node not saved.
I have several node types with mercury editor and mercury editor live edit enabled.
However, I have one node type that is too field-ish and I don't use Mercury editor or live edit there.
When I try to save the node, the form action leads to a path
node/4/form_action_p_pvdeGsVG5zNF_XLGPTvYSKCf43t8qZYSwcfZl2uzM
which gives "Page not found" and the node doesn't get updated.
Apparently the method "attachFormAttributes" in MercuryEditorLiveEditModuleHandler does not check if Mercury editor is enabled on the content type and sets the attributes regardless.
Adding a check with $this->isMercuryEditorActive($entity) before that is done fixes the problem for me.
Add the check.
Active
1.0
Code
I encountered this issue as well and the proposed resolution worked for me.
I made a PR for the update. Here's a patch in the meantime.