client: the math on this investment calculator you made is wrong.

me: ok, how is it wrong?

client: one of our salesmen sais it not calculating correctly.

me: that's fine. i just need an example, or the corrected formula to use.

client: on this other website we put in the same information and it comes out different.

me: ok, let me investigate... this other site adds a fee every month so the output is different. If i turn that off the calculations are the same. would you like to add a monthly fee field?

clients: no, the calculator is working how we want then.

repeat 5 times at 3 month intervals.

client: the interest calculator is broken again. didn't we just fix it?

me: it was never broken. your people just can't math.