The bus company made one change to the winter timetable. Every departure from the central station would leave ten minutes later than before. Nothing else was altered: the same number of buses, the same gaps between them, the same routes.
On paper it was the simplest change the planner had ever made. She did not redraw the timetable. She took the old one and added ten minutes to every line.
The shape of the service was untouched. If two buses had been twenty minutes apart in the old timetable, they were twenty minutes apart in the new one. The busy period still lasted the same two hours; it simply began ten minutes later and ended ten minutes later.
What surprised her was the complaints. They did not come from people who had lost a bus. They came from people at one particular stop, where the new departure time now fell just after the school bell instead of just before it.
The shift was the same everywhere. Its effect was not. A change that moves everything by the same amount can still land differently, because the world outside the timetable did not move with it.
She checked the other forty stops before she decided anything, and none of them had the same problem. The fix had to be as narrow as the fault.
She kept the ten minutes, and moved that one departure back to where it had been. It was the only line on the page that no longer matched the rest.