SCP-178 is a pair of ordinary-looking paper 3-D glasses. Putting them on reveals beings around you that are invisible to the naked eye. The trouble is that those beings notice the wearer and kill them instantly on contact. None of the entities appear on recorded footage at all.
The heart of the horror is that danger begins the moment you look. A threat that would not have existed had you not looked becomes real in the act of observation. The structure — unconfirmable by recording equipment, with only the one who looked placed in danger — is a variation on horror in which observation itself summons the threat.