Australia's most widely circulated banknote, the $50 has a spelling error. And it took about 2 months for it to be noticed. The word responsibility is missing an 'i'. Approximately 400 million of the banknotes entered circulation in October 2018.
According to an article at CNN:
The mistake -- which would be hard to spot with the naked eye -- is in microprint beside Edith Cowan, the first female member of an Australian parliament. The microprint next to her features excerpts from her maiden speech to the Western Australia state Parliament.
The RBA (Reserve Bank of Australia) said it became aware of the issue last December, and the spelling would be corrected for the next print run around the middle of this year.
"These banknotes are legal tender and can continue to be used as normal. It does not affect their validity and functionality in any way," a spokeswoman said in a statement to CNN. "We have reviewed our processes to remove the likelihood of such an error occurring in the future."
An updated $20 bill is expected later this year. I would assume it'll be checked very closely before being released into ciruclation.
Image Credit-Peter Parks/AFP via Getty Images via Bloomberg