I didn't see this as a "stupid things people do", rather, a "strange" things people do, thread.
One strange thing from my stint at a boarding barn, a lady wouldn't let her horses have any kind of salt or mineral block because "1. It had arsenic in it 2. It is nothing but rust, which is why it is red 3. It has a whole bunch of inhancers that make horses addicted to it. So they eat enough of it that the low amounts of arsenic eventually kill them."
Lesson learned. Salt kills. Lol