Up until a point way too far into my life to admit, I thought black markets were actual physical places in every city, and was completely baffled as to why they could continue to operate if they were both so wildly illegal and so widely known.
Then, having boatloads of free time and an over-active imagination, I proceeded to "invent" that which basically amounts to how this stuff actually works in the real world.
Once I found out about it, I sat still for minutes on end trying to figure out whether or not this means I'm a criminal mastermind, or criminally stupid.
I also thought that when people died in TV or Movies, the actor would actually die (this led to the conclusion that Sean Bean is a cat. Seriously).
I thought that when males turned 18, thus becoming legally adults, they would "grow" a suit, since kids didn't tend to wear them.
I also thought that the titles associated with parenthood would be passed on. Meaning a mother would only be a "mother" as long as her daughter gave birth, at which point the daughter would be the "mother", and the mother would cease being the "mother".
I had no grasp of time-zones or climate regions, so if it was 12:00 am where I was then it must be 12:00 am everywhere. Same with weather phenomenon. Because of this, the first time I flew from europe to australia, I was convinced we traveled through time.