What are some cool psychological phenomena?

If you stare at a pattern where the horizontal stripes are red and the vertical stripes are green, you might forever see horizontal & vertical stripes as having color even when they don't.

https://en.wikipedia.org/wiki/McCollough_effect

Sheperd tones. Sounds that appear to either rise or fall forever https://youtu.be/sjCLyi8bdBA

The backfire effect. This is when someone see irrefutable proofs that he's wrong, he believes more than ever that he's right.

https://www.skepdic.com/backfireeffect.html

Creating the feeling of body ownership. Trippy and makes me jump every time.

https://m.youtube.com/watch?v=lYQLFl-hgts

https://en.m.wikipedia.org/wiki/List_of_cognitive_biases

I am a big fan of the McGurk effect: https://youtu.be/2k8fHR9jKVM

Uncanny Valley: https://en.wikipedia.org/wiki/Uncanny_valley

tl;dr: Neither regular, boring, human faces nor highly stylized comic / chibi / animated human beings are percieved as "inhuman", but as soon as an artificial face looks too real but still slightly off, basically the middle of the scale between "real" and "definitely man-made", it can provoke feelings of extreme discomfort in the viewer. You're basically creeped out by something that tries to look human but fails at that only barely enough to notice.

Herd mentality

Any normal human being can be a bloodthirsty beast when he's a part of a group. The group effect demultiplicate the violence of their members and can lead them to acts that they would never do when isolated. Really terrifying mechanism.