The chemistry textbook: "Atoms always share electrons equally in a covalent bond."
Electronegativity: "Hold my periodic table."
Just like that school bus thinking it's safe on the tracks until the exception train comes barreling through, chemistry rules look solid until you hit chapter 8 where suddenly everything you learned is "actually, it's more complicated than that." Every chemistry student knows that moment when the professor says "remember all those nice rules? Yeah, forget those."