The 1844-O Half Dollar is one of the most dramatic engraving errors of the entire type. When creating the die, the engraver placed the date way too high in the die, embedding the top of the date deep in the base upon which Miss Liberty sits. After replacing the date, no effort seems to have been made to efface the original date, which shows u clearly in the field between the date and the base.
Most of the known examples are in circulated condition. According to the PCGS Population Reports (as of March 2012), PCGS has certified only two Mint State examples -- the finest of which is a single PCGS MS-64.