According to a Belarus Partisan article dated 23 February 2012, the 50,000-ruble (US$6.10) note which depicts Mir Castle has an error in the microprinting above the serial number at upper right on the back. If written properly in Belarusian, the name of Mir Castle should read МІРСКІ ЗАМАК (as it is written in large type in the caption on the front, as well as in the mircorprinted rectangle below the denomination at upper left on the front), but instead the text is written МИРСКІ ЗАМАК. Officials at the KGB and the National Bank of the Republic of Belarus claim that this is an intentional error intended to trip up counterfeiters, which may be true. Then again, it’s possible that the Russian engravers at Goznak simply made a mistake that has gone undetected for a decade.
This “error” appears on both the original 50,000-ruble note (B132a) issued in 2002 with solid security thread, as well as on the modified note (B135a) issued in 2010 with windowed security thread.
Courtesy of Vitali Khaletski.