<human>: Can you provide a brief overview of the text?

I'm a student from a small country where people are used to learning foreign languages. I've studied and gotten to use my languages with actual native speakers quite a bit, and one of the small things I love is noticing how differently languages are spoken compared to how they're taught in an academic setting.

Example: in high school everyone's mostly taught British English here, using all sorts of rules. One of those rules was that 'sick = nausea, vomiting; ill = a disease.' Then, when interacting with actual English-speakers (or American English-speakers at any rate), you find out that everyone just uses sick for everything and saying "I am ill" is tantamount to admitting you have cancer. 

Another example: in Spanish, I was taught proper grammar and punctuation. In particular, to start questions with ¿ (inverted question mark). Turns out, nobody in Spain ever uses that thing, ever, in everyday Facebook conversations and chats. I was told my putting it everywhere like a good boy was "cute."

Reddit, have you ever had an experience where you found out what you were taught was not the norm? Or have you ever corrected foreign speakers, telling them nobody ever said something like they just did, chuckling slightly but helping them prevent future mockery?
<bot>: a language you learn in school is taught using rules that native speakers hardly ever use, if they even know them at all. How have you messed up using these rules, instead of how the language is actually spoken?