What is just a placebo effect but most people don’t realize?

I don't know about any modern examples, but I remember back in the day, people swore that if you took a green permanent marker and colored on a CD with it, it made the sound better. They call it greening the CD.

It had absolutely zero effect on sound quality, because that's not how CDs and laser beams work, but the people who did it swore they could hear a difference in quality.