What is a well known 'public secret' in the industry you work in that the majority of outsiders are unaware of?

In HVAC, I've been called out to look at the air con in an office block and person A is cold but person B right next to them is hot, there's nothing we can do to help that.

One simple solution that seems to work each and every time is the placebo effect. I say ok, give me an hour, I'll adjust the parameters and check over the system operation. I then sit in the plant room for an hour paying on my phone, come back to see if it's better and 9 times out of 10 they're both suddenly happy.