I know Great Britain recently changed out their pound coins because a significant number were counterfeit.
Over the years, I've gotten a few under weight or just odd cents over the years, but have attributed them to wear or chemical exposure. I don't think that I have handled any counterfeit U.S. circulated coins.
There are known examples of silver coins, even to the quarter, so not meaning silver or numismatic coins. You know, that state circulation quarter is just such a keeper. Good enough to party like it's 1999.
Has anyone noticed any counterfeit modern circulation U.S. coins during your search?