An accident in 2366 that revealed a hidden Federation observation post to the Bronze Age Mintakans, who began to worship Captain Picard as a god until he showed them he was mortal.
The Mintaka III contamination takes place in the Next Generation episode Who Watches the Watchers. Federation anthropologists study the proto-Vulcan Mintakans from a concealed post, and when its reactor fails the holographic screen drops and a Mintakan named Liko sees the people inside.
Liko is injured and treated aboard the Enterprise-D, where an attempt to erase his memory fails. He returns home telling of an overseer called the Picard, and the old belief in supernatural beings, long abandoned by his people, begins to revive. A missing researcher and a captured Deanna Troi make matters worse.
Rather than let a religion grow up around him, Picard brings the Mintakan leader Nuria aboard and explains that his people are simply further along the same road. He finally proves the point by letting Liko wound him with an arrow, and the observers withdraw from the planet for good.