This seems to be the "fault" of Templating.php lines 153-160 ('catch' block) where if the request for the format failed, the content itself is changed to the message 'flow-stub-post-content'. This means that the recipient is unaware of the fact that there was a problem with fetching the data.
This is fine for viewing data (say, if a topic revision has failed, it will render on the page with that message) - but it is pretty unhelpful for the editing process, where the editor then simply displays that raw message in the editor as if that's the edited content. If a user clicks "save" on that, that will save and the user will have no indication that an error ever occurred.
- In a local wiki, shut down parsoid.
- Go to a Flow page.
- Edit the description/header
The editor opens as if nothing is wrong, with the content "Due to a technical error, this post could not be retrieved."