There is a compatibility issue between Gutenberg and classic wp-admin/Calypso for posts when:

• Created in the classic editor
• Gutenberg blocks are added without changing the classic block
• And the post is then updated in the classic editor again

h/t @KokkieH for the original report of this bug.

Steps to reproduce

1. Starting in the classic editor (in wp-admin or Calypso) create a new post.
2. Add some content to the post, and save or publish it.
3. Visit /wp-admin and selected "Activate Gutenberg."
4. Open the post in Gutenberg. (Note that the previously added content is in a classic block.)
5. Add a new Gutenberg block (e.g. a paragraph block) to the post and add content to it.
6. Save your changes to the post.
7. Deactivate Gutenberg on the site.
8. Open the post in the classic editor (in wp-admin or Calypso) again.

What I expected

I expected all of the post content to be there, including what I added in Gutenberg.

What happened instead

Only the post content added in the classic editor appeared. All of the content added in the Gutenberg block was gone from the editor in the visual or HTML view.

(In addition, in the classic wp-admin editor all of the content was collapsed into a single paragraph, regardless of the paragraph breaks that existed before.)

The front of the site showed all of the post content (including the content from Gutenberg) until I updated the post again in the classic editor, at which point all Gutenberg content was lost. I also did not see the missing content in the post revisions, so I couldn't recover it.

Browser / OS version

macOS 10.14.1 / Chrome 70.0.3538.77