Ninety-Ninety Rule

If

"The first 90% of the code accounts for the first 90% of the development time. The remaining 10% of the code accounts for the other 90% of the development time." The visible work hides a second, equally large body of integration, edge cases, and polish.

Then

When something looks "almost done," assume an entire second project remains: the last-10% of hardening, edge cases, and integration. Schedule it explicitly instead of treating it as a rounding error.

Unless

Demo on Real Data / Hard Definition of Done