I used to do programming for a company that did loan calculation software. We would get specs from insurance company actuaries for how to calculate consumer lo

I used to do programming for a company that did loan calculation software. We would get specs from insurance company actuaries for how to calculate consumer loans that were made with credit life insurance attached. The ##### actuaries would always send us formulas that looked like the annuity formulas in the article: annuity due = (1 - v^n) / d, annuity immediate = (1 - v^n) / i. Except, they did not notice that the formula fails for zero interest loans ( zero divided by zero), which were a popular sales gimmick. There are probably a million or more of those loans out there still getting paid off with payments calculated using some arbitrarily small interest rate selected by the programmer to eliminate the zerodivide error when the actual interest rate is zero. Some of the actuaries calculated the insurance charges using really fancy formulas that were not zero divided by zero when the interest rate was zero, but zero divided by zero divided by zero. Limited-precision calculations of these functions were very badly behaved in quite a large neighborhood of interest rate zero, so, despite all the professional refinement of the actuaries, there is no telling how erroneous the payment number on your loan documents came out.