I used to think "public convenience" meant a convenience store!
I used to think that in public toilets, the cleaning lady actually lived in the supply cupboard.
I was 6 and bored while taking a bubble bath when I discovered to my amazement that the back lid on the toilet tank could come off. I thought I had discovered a secret compartment that nobody else in the world knew about, and I proceeded to put all my bath toys into the tank for safekeeping. Needless to say, my father wasn't as enthusiastic about my secret toy chest discovery when I told him about it.
My gramma had a hole in the wall above her toilet and I used to believe a monster lived in there and would get me while I was on the toilet. So I always sat on her toilet backwards so I could see when the monsters were coming for me.
When i was younger, i believed there was an evil monkey who was hiding behind my bathroom curtains, and if i flushed the toilet, he would jump out and attack me!
My first rememberable experiences with urinals began at school, and those were the long metal ones designed for multiple people to use at once.
So when we went out for dinner one night, I went to the toilet, and for the first time I remember, I saw individual ceramic urinals, never seeing these before, I assumed these 'smaller toilets' were for 'babies' and the toilet cubicles/stalls were for grown-ups.
Later in the night I returned to the toilet shocked to see my grown up uncle among other people using the 'baby's toilets', I persuaded my slightly younger cousin, who also had to use the toilet, to use use the 'normal' toilet like a grown-up.
Nowadays - I coincidentally never use urinals, not because I believe they are the 'baby's toilets', but because I much prefer to do my business in privacy - even though I stand up when doing so.
When I was five or six, I used to pee in the drain at school. The reason being that I saw some boys doing it (I'm a girl) and thought it must be allowed. The Finding Nemo line "all drains lead to the ocean" kind of reinforced that belief.
I used to believe that the handicap symbol with the man sitting on the wheelchair was a man sitting on a toilet so I always got confused when I saw the handicap parking areas.
As a child I believed that toilets were alive. When you used them, you were feeding them, and if they didn't like what you fed them, they'd vomit it back up (overflow). If you made them angry, they would eat you, so you had to be nice to them and not make them sick. Stranger-toilets (e.g. public restrooms) were especially untrustworthy - best to keep an eye on them at all times.
I also believed that they would detach from the wall and dance around the bathroom at night when no one was around. You couldn't normally catch them at this, because they'd go back to being inert as soon as they heard you coming, but if you closed the bathroom door and then quietly crawled up to it and looked under the bottom, you could see this happening.
When i was a kid(about 6 or 7) I thought if you didnt get up before flushing the toilet, a monster would rise out of the toilet and eat you