Starting in the 1980s, The New Yorker magazine began responding to readers' letters under the name "Owen Ketherry," supposedly because the responses were the product of many editors. I found this out only today, when I Googled the name of Ketherry to find out more about the author (as I thought) of the following letter, sent to Mary Lutyens and included by John Julius Norwich in one of his excellent commonplace anthologies.
October 29, 1987
Dear Mrs Lutyens,
Thank you for your note, and for reading our pages so attentively. It is always a pleasure to hear from a well-informed reader although in this case the pleasure is bittersweet.
You are, of course, quite right about the quotation from Samuel Butler. We do pride ourselves on accuracy, but you know what pride goeth before. The methods we take to avoid mistakes are unusually thorough, but not, as you see, foolproof.
It is particularly mortifying to have made an accusation of error which is itself erroneous. It is the kind of mistake that can only be made in presumptuousness. We are chagrined, we are contrite and we are genuinely grateful to you for correcting us.
PS: We cannot answer your question about the number of readers who spotted the error, because their letters are still pouring in every day.