Shawn Spencer is on a SG team. Eventually the rest of the team learns to listen to him when he tells them to go back to the SGC rather than staying on a planet. (although possibly it takes him a while to pick up enough of the general customs/culture of the galaxy to be completely certain)

Backstory: he encounters Teal'c or another alien and actually says that they're an alien. I think he doesn't really believe it, but it's the only explanation that fits, so that's what he says. Then the SGC is all "oh crap" and takes him in for interrogation, where he eventually tells the truth.