There was also a well-publicized study in which the a treatment group of smokers were given vitamins A and E as an attempt to reduce lung cancer. They actually got it more often. The trial was stopped.
Did the vitamins cause cancer? Probably not. A change in other behaviors is more likely.
In a recent HIV vaccine study, those who got the vaccine actually got HIV more often. This trial was also aborted. Since it was biologically impossible for them to have gotten the virus from this vaccine, researchers eventually concluded that they were just taking more sexual risks.
When people think they are protected by something, they stop trying as hard to protect themselves.