I created a test environment in which I created three users.
User A has Mozaik as preferred email editor
User B has TinyMCE as preferred email editor
User A has Direct HTML Editing as preferred email editor
First I logged in as User A and created a new email template with Mozaik with a few blocks.
Then I logged in as User B and edited the template (just added a few words at the top without any div or incapsulating tags) (TinyMCE)
Then I logged in as User C (plain html editor) and I added a span with a couple of words in the middle of the email template. The span is correctly placed inside an existing div.
Then I logged out and logged in as User A (default: Mozaik).
At first sight everything seemed normal, but, when scrolling down I realised that the whole email was repeated three times (probably one for each edit)
Then I edited (with Mozaik the code and, as soon as I entered the Mozaik editor all the edits made with the other users disappeared as well as the two additional copies of the original mozaik template.
Additionnally I checked the html code (before the last edit with Mozaik) and I noted that there were a number of html input tags that had been added.
I believe that all these are serious issues. Unfortunately I don’t know how to contribute towards solving them. I just want to highlight it because I don’t think that anyone who puts this in production and uses email templates will be happy to see his templates damaged in such way.