There is a large and strong group of collectors who search for coins, stamps and banknotes which contain errors; the crazier the error, the better. The rarer the error, the better. One of the most famous collectable errors is the 1918 “Upside Down Jenny” US Postage Stamp.
At one point, an unused version of this stamp sold for $1.6million. The value of errors usually comes down to their rarity. Only 100 of these were believed to be made. If you are an error stamp collector, this is the Holy Grail. Rarity is one part, but I believe the uniqueness and mistake during production appeals to the detail-oriented personality of many.
Polymer and paper money manufacturing involves dozens of quality checks, so the possibility of a piece of currency slipping through and ending up in circulation is indeed uncommon. Recently, the 2018 Australia $50 (P-65a) polymer note came out with an incorrect spelling of “responsibility” in microprint. There were 46 million printed, and the mistake can only be seen with a magnifying glass. It is still collectable because people like to have errors. Money with errors in denomination is exceedingly rare.
Nigeria also had a spate of errors. There are a number of 10 Naira P-39c from 2011 banknotes that have been printed with mismatched serial numbers. Serial numbers are like a banknote fingerprint, proving authenticity. To have mismatched serial numbers that are not caught during printing seems pretty unusual and interesting.
Miss-cuts are also considered errors, and they can be great talking points when a collector shows their notes to a friend especially when the cutting dramatically changes the look of the banknote. Extreme errors like the example on Banknote World of a 2004 Zimbabwe 500 Dollar P-11b where the miss-cut has pieces of 3 banknotes in one example.
Other examples that are very interesting are errors where a significant part of the image was not printed. There is a 2008 Zimbabwe 100,000 dollar P-75 that was printed without one of its offset ink colors is very unique and easy to explain what went wrong. Finding ones that are graded or TAPPED is even more atypical, more valuable and more fun.