If a user accidentally publishes a version consisting of the other person's text, their own edit is completely gone.
If an editor publishes a version that only consists of the other person's text, add a confirmation before saving. This could be along the lines of "You're about to publish none of your original changes. Proceed?"
Note, this has been merged with "No warning if closing the tab or clicking cancel in TwoColConflict", we should either unmerge the tasks or consider the abort cases here.