Two personal examples of "you don't know what you don't know" come from British kidlit. As a child I read "The Secret Garden" and pictured the #robin in the book as identical to the ones I saw every spring, because same name, same bird, right? Only a few years ago I discovered by chance that American and British robins are completely different species. (Australian robins are also a different species, unrelated to either American or British robins.) I didn't think to ask, because I thought I knew. (more)
Another: in "The Last Battle," King Tirian of Narnia shares food with two children from our world. The book describes two egg sandwiches, two cheese sandwiches, and "two with some kind of paste in them" which the king probably wouldn't have eaten if he weren't so hungry. As an American child I assumed the paste was either tuna salad or peanut butter. I'd never even heard of #Marmite, if indeed that's what Lewis intended it to be. It could be some other British sandwich filling of which I have no inkling.

@Soozcat Funny, this cognitive stumbling block pinged me yesterday… talking with someone about one of the Afroman songs mocking the Adams County Sheriffs' escapades, I learned part of the chorus is actually "Fix my do' [door], stop stealing my dough, stop disconne-cting my video".

All I initially parsed from the first three sounds was "?ihss? my doe", so my brain filled in the gaps to "It's ~my~ dough, stop stealing my dough". Since I 'knew', I didn't think to look further into it.

@Soozcat
All you ever wanted to know about meat and fish paste.
These are what were a staple sandwich filling for UK kids in 1900's and during wartime.
Not as bad as you may think, even cheese was scarce and rationed then so school sandwiches were mostly these pastes.
*Sorry link failed try google Shippams paste

https://www.thenovium.org.uk/article/28861/History-of-Shippam-s

@RHW My mother was born near the end of WWII, and one of the sandwich fillings she remembered fondly from her California childhood was "olive nut," a paste made of chopped black olives, chopped walnuts and a hefty amount of mayonnaise. She made some a while back for me to try, and... let's just say it was an acquired taste.
@Soozcat I well believe you. 😊😊