What is the silliest thing that you have heard somebody say they believe?
First, any mention of religion of any type is off topic as this question is not about those types of beliefs.
Instead, it is about those little curiosities that people you know (or knew) believed to be true.
I will give an example. A girl I went to university with believed that if you visited another country for longer than 24 hours that you legally became that nationality. This would mean that if I was on holiday in Spain she believed it meant I was then legally Scottish (since I am), but also part Spanish. If I then travelled to China I would be Scottish, Spanish, and Chinese and so on. It was her belief that you could get a passport for the country and that due to being in the country you were a legal citizen.
So what strange things have people around you said that they believe in?