According to an article on the Belarus Partisan dated 05.11.2010, on 1 September 2010, new rules of Belarusian orthography came into force. According to the old rules, the correct spelling of the word “fifty” in Belarusian was “пяцьдзЕсят,” but under the new rules, it should be spelled “пяцьдзЯсят,” the difference being that the seventh character was the Cyrillic letter IE but is now the Cyrillic letter YA. As a result of these new rules, the existing 50- and 50,000-ruble notes (B125 and B132, respectively) dated 2000 now technically contain errors where the denominations are spelled out on the backs of the notes. Belarus National Bank intends to correct these spelling errors when it issues new versions of these denominations, but there is no word yet on when that may be.
Courtesy of Vitali Khaletski.