It seems that the US Mint makes a mistake occasionally. That means some fairly valuable U.S. error coins are in circulation. But these errors are not readily evident and require a 4x or 6x magnifying glass to be seen.
Is it worth the time and effort? Well, there are some coins still in circulation that can fetch up to $35,000 on the collectors market.
More information is at this link or here's another one and this link has pictures, no less.