The 404 was treated as

The 404 was treated as a proper page rather than a fallback state.

The number is constructed as an architectural drawing. The 4 and the 0 are formed from walls at true thickness, with furniture laid out inside the resulting rooms and dimension strings running along both edges. It sits inside the same drafting system as the rest of the site rather than borrowing a display typeface.

The error message then repeats. Instead of a single dialog, identical windows cascade back across the canvas, each one carrying the same 404 notice. It reads as a system throwing an unresolved warning over and over, which is the exact behaviour Heron is built to prevent inside a model.

Error pages are never in the brief. They're a good measure of whether a team understood the product or just styled it.