A common belief I disagree with is that you need to be naturally talented at maths to succeed in it. I'm a twenty-eight-year-old maths tutor in Karachi, and I hear parents and students say 'I'm just not a maths person' almost every week.
It's held by a lot of people, but particularly by parents who struggled with maths themselves. They often say it in front of their children, almost as a joke, and the children pick up that idea very early. Some teachers believe it too, especially when they divide classes into 'strong' and 'weak' groups at a young age.
I think people believe it because maths can feel so black and white. In other subjects, you can get partial credit for a decent answer, but in maths, if you make one small mistake, the whole answer is wrong. So children who fall behind early feel stupid, and that feeling becomes part of their identity. It's also easier to say you lack talent than to admit you didn't practise enough or had poor teaching.
I disagree because I've seen the opposite many times. One student, Hamza, came to me at fourteen convinced he was hopeless. He'd missed a lot of basics, like fractions, so everything after that seemed impossible. We went back and filled those gaps patiently, and within a year he was getting top marks. Maths is like a ladder; if a few steps are missing, you can't climb, but that's not about intelligence. I'm not saying everyone will become a mathematician, but almost anyone can become competent. Believing otherwise stops people before they've even started.