Variables won't, constants aren't.
From the Murphy's law canon, and a warning about model inputs: parameters assumed fixed will drift, and parameters assumed to vary will turn out to have been fixed by something nobody documented.
In practice
- A model whose "fixed" exchange rate assumption held for eleven years breaks in month one of the forecast.
- A capacity plan built on a constant conversion rate is invalidated when the marketing mix changes, and the constant was never a constant.