Cecily. [Rather shy and confidingly.] Dearest Gwendolen, there is no reason why I should make a secret of it to you. Our little county newspaper is sure to chronicle the fact next week. Mr. Ernest Worthing and I are engaged to be married. Gwendolen. [Quite politely, rising.] My darling Cecily, I think there must be some slight error. Mr. Ernest Worthing is engaged to me.

–The Importance of Being Earnest,
Oscar Wilde

What literary device does Gwendolen use when she says, "I think there must be some slight error”?