Someone mentioned this above, but it bear repeating: a strong radiation source.
If you are using a standard film camera, radiation will “fog” the film long before you’ve been exposed to enough to make you sick, and while there are ways to protect the film from radiation, if you still want to be able to take pictures there are limits. (Glass with lead in it isn’t lead, if it lets in light it will also let in radiation, etc.)
If you are using an electronic camera, again long before doing you long-term harm the radiation will flummox the camera. It may be easier to harden the camera to radiation than with film, but … in really heavy radiation exposure, the best we’ve got would last for a few minutes I believe. A human could take in the same view, and while he probably wouldn’t survive, he wouldn’t show symptoms for a while.
You can survive brief exposure to radiation strong enough to make you see fireflies (the particles hitting your retina make little flashes of “light” in your field of view), but that will ruin most any attempt to capture an image of what you are looking at.
I am sure there are some strong sources of radiation in the world that rely on most people not being suicidal as their primary line of defense, so if someone truly wanted to physically look at them one could. Whether any of them are strong enough to destroy shielded cameras is in doubt, but possible I think.
I was leaning towards the “holy site” folks: somewhere where anyone CAN go, but those who do have the respect not to photograph what they shouldn’t photograph, but … people are jerks, cameras are everywhere, and I agree we can’t say with certainty that no such picture exists.
Talk about the security used at … Lenin’s tomb, for example, doesn’t address the possibility, just the probability: I could get a hidden camera built into the frame of a pair of glasses, and I’m sure I could figure out a way to sneak some sort of camera past guards and get it pointed in the right direction for a while. The question is would my images have enough worth (monetary or otherwise) to make it worth all the effort. And in a lot of cases my answer to that is probably “no”. I just don’t see the National Enquirer paying big bucks for exclusive photos of Mao’s corpse, or whatever.