Ah. The other failure mode that occurred to me is that if a spread sheet is involved you could keep running the same calc on a bad input for months or even yea

Ah. The other failure mode that occurred to me is that if a spread sheet is involved you could keep running the same calc on a bad input for months or even years when aggregating intermediate values over units of time. A problem that happens every time you run a calculation is very different from one that happens at random. Better in some ways and worse in others.