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An example of a real-world visual infohazard that isn't all that dangerous, but is very powerful: The McCollough effect. This could be useful as a quick example when introducing people to the concept of infohazards in general, and is also probably worthy of further research by people smarter than me.
Huh, started reading about this and then sorta got scared and stopped. :P
(I googled "McCollough effect", and looked at a bunch of images for awhile before starting to read the article, then the article was saying "looking at the visuals might leave lasting changes", then I some combination of 'freaked out slightly' and also 'decided I didn't care enough to finish reading the article')