What's the most surprising thing a native speaker told you about your target language?

I've been learning Spanish for about two years, and recently had a conversation with a native speaker from Mexico who told me that a phrase I'd been confidently using for months actually sounds pretty unnatural in casual conversation. Humbling, but also one of the most useful moments I've had as a learner. Textbooks and apps can only take you so far, and real feedback from native speakers often reveals things you'd never pick up otherwise.

It got me thinking about how many small nuances exist in every language that just don't make it into standard learning materials. Regional expressions, subtle tone differences, words that are technically correct but sound awkward, phrases that have shifted in meaning over time.

Curious what surprising or eyeopening things native speakers have told you about your target language. Maybe something that changed how you studied, or a correction that made you rethink a habit you'd built up. Doesn't have to be embarrassing, just something genuinely unexpected that helped you as a learner. Would love to hear from people learning any language at any level.