On the other hand, sometimes mathematicians are mediocre programmers. Look at this "solution" to the subset sum problem: Subset Sum. He says to multiply out N binomials and collect the terms, which is O(2^N). Mathematically it's correct; computationally it's not feasible.

(I know that the subset sum problem is NP-complete, it's just the author of the article presents this as a solution without mentioning the issue at all.)