"The edit could not be undone due to conflicting intermediate edits."
In this case, the text of the old revision, identified by undoafter=185338 in the example, should be shown in another text area at the bottom, as with edit conflicts.
The user could then copy and paste the whole old text, or just the relevant parts of it.
Currently you need to edit the old revision and save for a complete revert (which shouldn't fail with undo if used correctly, though), or even to edit both the old and new revisions and to switch between the two if you need to copy and paste different parts. Very obvious for us, quite obscure for newbie. (Observed on #mediawiki a short while ago.)