[trca]

According to Hidden Figures (which to the best of my knowledge is quite accurate in representing Katherine Johnson), she actually hand checked most if not every calculation performed by the digital computers and often found miscalculations due to programming errors.

[mdiesel]

Digital computers were in their infancy - at one point in that brilliant movie the critical calculations are passed through the digital computer twice and got different answers. Katherine was asked to repeat the calculations and her result was then used.

The movie also shows Dorothy Vaughan teaching themselves how to program those first machines. Remember that pretty much noone had any programming experience, and there are still pitfalls today for people doing science calculations on a computer.