Next to a photo of Angelou reads the text, "A bird doesn't sing because it has an answer, it sings because it has a song."
Those words may recall the title of Angelou's 1969 autobiography I Know Why the Caged Bird Sings, but they were actually written by Joan Walsh Anglund, 89, in the 1967 book A Cup of Sun, according to the Washington Post.
"Had we known about this issue beforehand, we would have used one of [Angelou's] many other works ... the sentence held great meaning for her, and she is publicly identified with its popularity," Postal Service spokesperson Mark Saunders said in a statement reported by the Post.
USPS / AP
But the author hoped the item would be successful regardless: "I haven't read all of her things, and I love her things, of course," she told the Post.
She continued, "But I think it easily happens sometimes that people hear something, and it's kind of going into your subconscious and you don't realize it."
The Tuesday morning ceremony featured an impassioned speech by Winfrey praising Angelou, who died last May at 86. Her speech continued even when the power went out, rending the room completely without lights.
Michelle Obama praised Angelou as well. "She taught us that we are each wonderfully made, intricately woven, and put on this earth for a purpose far greater than we could ever imagine," the first lady said. "And when I think about Maya Angelou, I think about the affirming power of her words."