If you are going to make a mental note for penny errors the 1969 S DDO is the one to remember. The 1969 S Lincoln Cent Double Die Obverse is the most valuable Lincoln error in existence. You have a better chance finding this coin than any other coin previously mentioned on the list. Just 30 specimens are known to exist and a few were strangely enough destroyed by the government.
Like all other copper pennies, color and condition means most to collectors. MS-65 1969 S Red is the gold standard for this Doubled Die Obverse error. One sold for $126 thousand dollars back in 2008. While the most recent sale for this coin in the same color and condition is $50,000.
Most recent sales of the 1969 S Doubled Die Obverse cent sales. These are in order with the newest being on top.
The 1942 Aluminum cent was an experimental coin struck when the United States was looking for alternate material to be used with the 1943 cent. They eventually settled on Steel making this coin even more rare.
The edge is smooth like all pennies, but the planchet itself is much thicker than the current penny. This penny has been analyzed to be 98% aluminum. Despite having nearly 2x the thickness of a normal issue 1942 penny this coin weighs just 1.56 g compared to the normal 3 g of a copper penny Making it feel more like a token than a real penny.
Very few specimens are known to exist making this coin even more valuable.
This error is one of the "stranger things" you can find while searching for errors. So strange we should have made it number #11...sorry for the sci fi reference / dad joke.
All joking aside, this coin is a bit of an anomaly. The front or obverse of this coin looks like a regular issue 1999 1C coin that has had some lettering near the edge worn away. The reverse however is a bit off. The reverse is that of a 1999 dime.
This coin was found in a roll of 1999 cents picked up from a bank. Meaning there are likely more in circulation to be found. While this specimen is currently only 1 of 1 being from 1999 means there is a decent likelihood that others exist, and have yet to be found.