I’ve had this coin in my collection for awhile now. To me, it’s one of the most unusual errors in US coinage. The die engraver must have been distracted or hung over. These pics are under natural light. Just a beautiful design.
It took me a while to figure out what you are talking about, but yes. That's a cool error. Someone started engraving "United" under the 50 C. @Treashunt
Is that the one known as the 50/UNI reverse? If so because it is on the die it would be classified as a variety not an error, but still a beautiful coin