The Herod Effect is named after a joke I once made that quickly got out of hand. My sister is a very Christian person and named her first two children after characters in the Bible. When she had another baby I told my wife that she had decided to call it Herod.
I didn't think for a moment that she'd believe me but it turns out she did and told everyone she knew, who also believed it. Thanks to social media the story spread across a surprisingly large fraction of the globe. It is fortunate that when my sister found out (I've no idea how) she saw the funny side.
The Herod Effect is responsible for a lot of privacy violations. False stories can be harmful in many ways, including undue scrutiny. Everyone my age remembers the false story about Richard Gere and the gerbil (look it up on Snopes if you're young). I expect people liked this story because it allowed them to feel superior in some way to an otherwise esteemed celebrity. Sour grapes. It's a story people want to believe because they are unthinking, horrible pricks.
I've no idea whether Gere's career was harmed by this story but he - and his sex life - certainly came under huge amounts of scrutiny. It was suddenly fair game to pry into every aspect of his life because a rumour, buoyed by The Herod Effect, caused people at large to feel that another human's private life was somehow in the public interest.
There are plenty of examples of non-celebrities being harmed by the Herod Effect. The one that comes most quickly to my mind is that of the games journalist Zoë Quinn and Gamergate, False claims about her sex life and ethical practice led to years of misery and countless violations of her privacy including doxing, as well as countless threats of rape and murder.
It was a story that some people (specifically horrible, bigoted people) wanted to believe regardless of facts because it seemed to confirm their biases. That the story was about the supposed sexual impropriety of a woman, pitched to people who already felt that women have no place in video games journalism made it irresistible to many. Very little critical thinking took place in a community of people who tend (quite wrongly) to pride themselves and each other on their rationality.
So that's The Herod Effect. We've all been guilty of it at one time or another. And we shouldn't. We should check our facts because the effects on privacy (and on well-being in general) can be horrific.