Hardcode

A number typed directly into a formula rather than referenced from an input cell.

It is the most common defect in financial models and the hardest to find, because the cell looks like a calculation and behaves like a constant. Six months later nobody remembers why the growth rate has a 1.02 buried in it.

The convention that prevents it is colour: inputs in one colour, formulas in another, and no exceptions.

Every assumption gets a cell with a label.