One of the explanations in the irrationality game thread for UFOs and other paranormal events seen by multiple people at once, like the was mass hysteria. This is also a common explanation given for any seemingly paranormal event that multiple people have independently witnessed.
But mass hysteria is mostly known from incidents where people hysterically believe they have some disease, or have some hysterical delusion (false belief). In cases where people report seeing something or having a hallucination, it tends to be a few people across a large society. For example, when reports of Spring-Heeled Jack were going around England, multiple people claimed to have seen Spring-Heeled Jack, but there were no cases of hundreds of people seeing him simultaneously; therefore, the hysteria could have selected for people who were already a little bit crazy, or it could just have been that out of millions of English people a few of them were willing to say anything to get attention.
Conformity pressures can cause people to misinterpret borderline perceptions - for example, if someone says a random pattern of dots form Jesus' face, I have no trouble believing that, thus primed, people will be able to find Jesus' face in the dots. But it's a much bigger leap to assert that if I say "Jesus is standing right there in front of you" with enough conviction, you'll suddenly see him too.
Does anyone have any evidence that mass hysteria can produce a vivid hallucination shared among multiple otherwise-sane people?