In 1965, a scientist named Celeste McCollough discovered an unusually powerful optical illusion. I recommend trying it yourself; it is very subtle, but lasts so long it's eerie.
A website of instructions and background gives surprising details about the illusion:
It has been the focus of on-going investigation ever since [its invention in 1965].
The effect typically lasts for hours, or even overnight. The duration can be changed by the consumption of coffee and other psychoactive drugs. One paper found that it is stronger in extroverts than introverts, and might be a reliable test for extroversion.