The Warren Silver Cup is a most controversial item. It depicts pederasty in two scenes on both sides. An onlooker - likely a servant - can be seen peeking through the door at one of the pairs of males engaging in homosexual activity.  

The cup is labelled a forgery by certain critics. It is said that the style of iconography is not Roman, but rather 18th or 19th century. They also say that the fact that it's silver makes it more likely to be a fake as Roman silver is rare. 

It sparked a whole debate amongst Romantic scholars in the 19th century about its authenticity, some of whom cite it as a proof and demonstration of the supposed normalcy of pedophilia and pederasty in Greco-Roman times. 

I am no expert on Roman cups, but the inclusion of the onlooker peeking into the room seems a bit of a red flag to me - as Romans were not known to be couched about homosexual activity. The onlooker provides a bit of sleazy whimsy which doesn't seem authentic. Also two scenes of pederasty on one cup seems suspect, as it changes what would probably be cultic iconography - as would be common on any decorative vessel - into nothing more than simple pornography. 

However, pornography was commonplace in Ancient Rome, as is obvious to visitors of Pompeii, and the artistic style is neither here nor there in respect to its potentially being Ancient Roman versus Romantic-era English / French / Italian, so there is really nothing barring The Warren Silver Cup from being a true artefact.