(Of an occurrence) become as though never having happened; be reversed:things had happened that could never unhappen...
To paraphrase P.J. O'Rourke, unapproved interactions unhappen in Pyongyang.
The truth of the proposition is based on the fact that there has been a case of my going to Athens, and - whatever happens from now on - this cannot unhappen.
But her blissful countenance made every bad thing unhappen.