There is no obvious way to undo header vandalism, such as what happened recently to Talk:Beta Features/Hovercards. The only way I could figure out is to copy the text from the diff into the header edit box and then edit it to get just the before text, which is very inconvienent.
Topic on Talk:Structured Discussions/Flow
Jay8g: Yes, it seems to me those who can edit something should be permitted to Undo. Should Flow treat header specially? Topic title editing? Also, the RecentChanges feed could have a [Rollback] link for those with the user right.
S Page (WMF): I suppose this applies equally to anything publicly editable (header, thread title, thread summary...)