“Sight Unseen” is episode thirteen of season six of Stargate SG-1.
SG-1 come back through the stargate with an artefact from what they believe is an Ancient science station. Carter says that she thinks the device is harmless (having watched Stargate: Atlantis I’d hesitate before saying any glowing Ancient tech is harmless – the Ancients could be a bit erratic when it came to technology). At which point, Jonas starts seeing things that no one else is. At first, they’re concerned that it may be related to Jonas’s exposure to naqadria. but then others start seeing them too.
It seems that the device may simply be allowing people to see creatures that exist in another dimension. Technically harmless, unless everyone starts seeing them, at which case panic could ensue. Of course, the device doesn’t seem to have a simple ‘off’ button. Although they do manage to sever power to it. Naturally, that doesn’t work, so they take it back. That doesn’t work either.