this is a conversation going on at FORVM and I figured I would answer it here--
"Again, in the hundreds of years that Roman mints struck coins, is it possible that at one time or another mint officials made counterfeits? Yes. I suspect it was extremely rare, if ever. So rare, that one should not speculate any particular coin is an unofficial product of an official mint or mint worker without very good proof. I have never seen such proof and I think I never will."
there is no answer to how often this happened, but proof that it ever happened, or at least the Roman government believed it was happening, can easily be found in the Theodosian Code.
"Since some imperial minters are secretly and criminally engaged in the coinage of counterfeit money..." Theodosian Code 9:21:2 (20 November 321)
