There are cases that people know that a certain naive mathematical thought is incorrect but largely overestimate the amount by which it is incorrect. I remember hearing on the radio somebody explaining: "We make five experiments where the probability for success in every experiment is 10%. Now, a naive person will think that the probability that at least one of the experiment succeed is five times ten, 50%. But this is incorrect! the probability for success is not much larger than the 10% we started with."
Of course, the truth is much closer to 50% than to 10%.
(Let me also mention that there are various common false beliefs about mathematical terms: NP stand stands for "Nondeterministic Polynomial time"and not polynomial" [in fact it stands for "not polynomial"Nondeterministic Polynomial" time]; the word "Killing" in Killing form is an adjective [in fact it is based on a the name of the mathematician "Wilhelm Killing"] etc.)
