As a child I believed that math was the study of what, exactly, it takes to break the hearts, minds, and souls of children.
It nearly killed me to get though Math 111 in University. Never have I studied so hard for a B... and I say that as someone who took a medieval French literature class in French. (In complete honesty, I'm not that bad at math, I just have severe text anxiety and all of my math classes were graded entirely via exam performance.)
So I always derived some comfort from the fact that Albert Einstein had also struggled in math at school. This was, in my young opinion, proof of how messed up the school curriculum of judging one's ability to do something in a high stress environment (and the dreaded "answer as many questions as you can in 5 minutes" mini-quiz torture). His failure was my validation that math as it was taught in school was utterly useless.
You can imagine my disappointment when I learned that Einstein had never actually had any problems with math in school.
So why do people think that Einstein actually failed math in school? Luckily for us, there's a pretty obvious smoking gun, a Ripley's Believe It Or Not:
I'd advise that you don't believe it, since in 1935 Einstein laughed when he was shown a clipping of the Ripley piece, saying, "I never failed in mathematics. Before I was fifteen I had mastered differential and integral calculus." [*]
Where did Ripley's get the idea? Probably a combination of things. First of all, people really like the idea of someone struggling in something, and then becoming the best at it because of their struggles. It's just appealing.
Secondly, it's likely that the school Einstein attended changed their grading system, which made his previously high marks look unimpressive. For example, if the highest grade possible is a 6 in a scale of 1 through 6, and then the system changes to a score of between 1 and 20, suddenly that perfect grade of a 6 looks pretty horrible. [**] A lot of schools tried to standardize their scoring for grades during this time, but not a lot of records remain.
Lastly, Einstein was really good about having others check his math before he published. Which is really just a good idea, because it's just as easy to make a typo in math as it is in any other written media, but also made people think that possibly he wasn't good at math. [***]
In reality, Einstein almost became a mathematician instead of a physicist. He just decided that physics were more interesting, but still got himself certified to teach math. Incidentally, he did fail one subject that many other have struggled with -- French. [****]