Mass hallucination

[Mister Agenda]

Too many, I think that would more properly be a mass delusion.  

What's the difference?

If I see a squirrel that isn't there, that's hallucination. If I am convinced squirrels are secretly more intelligent than humans, that's delusion. It's a difference between a perception and a belief. If I believe the earth is flat because I was an astronaut and that's what it looked like to me while I was in orbit, that would probably be an hallucination. If I'm convinced the earth is flat despite having been in orbit and directly experiencing its oblate spheroidness, that's a very strong delusion.

Interestingly, a person can experience hallucinations without also having the delusion of believing they're real. I would consider that a good evidence of something being a mass hallucination: everyone saw it but some people realize it was hallucinatory.