I recently read an interview with magician James [The Amazing] Randi. He said: “You know the expression, ‘warts and all?’ Oliver Cromwell, I believe, was supposed to have said that.” I decided to take a look.
“Warts and All” means to show something in its entirety, even its unattractive aspects. Phrase Finder says the saying is attributed to Cromwell as his instructions to Sir Peter Lely, who was painting a portrait of him. However “there doesn’t appear to be any convincing evidence that Cromwell ever used the phrase ‘warts and all’. The first record of a version of that phrase being attributed to him comes from Horace Walpole’s Anecdotes of Painting in England, with some account of the principal artists, 1764… We can only assume he was indulging in a piece of literary speculation.” Somerset Maugham used the same phase and attribution in his 1930 book Cakes and Ale
English for Students reproduces the same origin almost word-for-word as Phrase Finder. I didn’t find any source to dispute this, but it does illustrate one problem with looking for word origins. Many sources are not independent.