I have some of those too :D

• When i was young i helped my mother quite frequently when she was cleaning the house even though i wasnt really helpful at that age :D Anyways she told me that those dishwasher tabs are so toxic that even just touching them could poison you bad. In retrospect i know that she just tried to scare me so wouldnt accidentally swallow the tab or put the finger which touched the tab in my mouth but i somehow never questioned it until a few years back when i asked myself why i always tried to have the least possible contact with it
• I thought that my pokemon games back then were connected to the internet or some other network because i kept discovering new pokemon and was convinced that they werent in the game before :D Most of the time i even just read about them in a megazine so i kind of get why i came to the conclusion even though it seems just stupid today

Dont know if any of them count as serious because i usually just remember those which i thought would be the dumbest :D