"Intentional" use?

A US baseball star who told Congress he had never taken steroids has been banned for 10 days for violating the sport's anti-drug policy.

How, exactly, does a person use steroids unintentionally? An athlete is someone whose body is a tool in his profession. One would assume that as an athlete's pay is linked to his performance, he would be hyper-aware of what substances entered his body. So how would a substance, such as a steroid which typically requires injection, "sneak" into his system without his awareness? How could he "unintentionally" injest steroids?