I used to believe (until very recently!), that in an airline safety demonstration when they said "the inflatable exit slides can also be used as floatation devices in the event of a water landing", that they helped keep the plane afloat!
A childhood belief about aeroplanes

others believed things like this
- My friend's sister used to think the auto pilot on a plane was an inflatable man like the one in Airplane!— Richard Davies
- Up until I was in 3rd grade I believed my bathing cap is what kept me afloat in the swimming pool! Anon
- i used to believe the turning signals on cars helped them turn sharp corners.— Julia
- I used to believe in the plane there were birds to make it fly— laura 401
- I used to believe that Florida was up in the clouds and that's why we had to take a plane to get there.Anon
- I thought that hydrogen was pure water (since I knew hydro- meant water) and H2O was impure some how— Hexetic