According to the Royal Mint, 3% of pound coins in circulation are fake. I got my first fake pound coin--one that I identified as such, at least--a couple of weeks ago. (I'll post photos when I get a chance.)
In the modern world, where the metal value of circulating coinage has little relation to its face value, counterfeiting to capture the difference becomes attractive. Once a circulating coin has a high enough face value, it starts to be worth counterfeiting. Empirically, that value appears to be somewhere above $1, as the pound and two euro coins are both among the most counterfeited coins.
Circulating coins aside, coin collecting has in recent years been dealing with a wave of fake coins produced in China for the collector market. While it hasn't been particularly cost-effective to make fake American coins for circulation, it has definitely been worthwhile to fake collector coins.
In the same way modern laser and inkjet printers have made it necessary to incorporate new security features to paper (and polymer) money, 3D printers will drive the adoption of security features to circulating coins. It may also drive the incorporation of security measures like the EURion constellation recognition features into 3D scanning and design software.
