Ask HN: What facts did you learn that you can never un-know?

Looking for examples of relationships in the world where once you see or learn them, you cannot un-see them whenever you encounter them.

To me more advanced ones include things like interest compounding, standard negotiation and sales tactics, pareto distributions, and survivor bias, but they could be as simple as proverbs, heuristics, similes and metaphors, and even poetic descriptions.

What are other examples of these frames that, once you view the world through them, they stick on your short list of go-to mental reference points?