According to Augustine: A New Biography by James J. O'Donnell,
Samuel Beckett tormented his interpreters with a story he told one time, supposedly to help them understand Waiting for Godot. He used to read, he said, a lot of Augustine in the Bibliothèque Nationale in Paris and recalled one passage in particular: "Do not despair," he remembered Augustine saying, "one of the thieves was saved. Do not presume, one of the thieves was damned." Thinking of Vladimir and Estragon as the two thieves crucified with Jesus is intriguing, to say the least, and is wonderfully Beckett-like that the particular passage cannot be found anywhere in the surviving writings of Augustine or anywhere in the pages of Patrologia Latina, for all that language and tenor are quite perfectly Augustinian. Did Beckett make up the quotation? Is he the most modern of pseudo-Augustines?
When searching around, the quotation is usually appended to Augustine and it is through Beckett that it all started, though it is a growing consensus in academia that it was not Augustine that penned the phrase but Beckett.