What's wrong with making design changes at the end of the project?

Expensive and time-consuming

When the addition of hardware fails to solve the problem then design changes are needed to rectify the problem. Unfortunately, these design changes now need to be made at a point when time is critical and customer expectations are high. However, there is only a limited amount of tuning and redesign that can be made to a fully built, functional system. This can be likened to modifying your house to be energy efficient - you can added double glazing and loft insulation but it will never be as efficient as a house designed from day one to be energy efficient using energy saving materials, low ceilings and large windows on south facing aspects.