In the US there are a huge number of banknote mules. My guess is, the reverses were so similar, they just used them with the new obverses until they were too worn to use.
India also had mule banknotes in 2014 - with the signature of D. Subbarao on them instead of that of Raghuram Rajan, who became Governor of the Reserve Bank of India in late 2013.
