If the error is made and the consequent defect is detected in the requirements phase then it is relatively cheap to fix it.
Similarly if an error is made and the consequent defect is found in the design phase then the design can be corrected and reissued with relatively little expense.
The same applies for construction phase. If however, a defect is introduced in the requirement specification and it is not detected until acceptance testing or even once the system has been implemented then it will be much more expensive to fix. This is because rework will be needed in the specification and design before changes can be made in construction; because one defect in the requirements may well propagate into several places in the design and code; and because all the testing work done-to that point will need to be repeated in order to reach the confidence level in the software that we require.
It is quite often the case that defects detected at a very late stage, depending on how serious they are, are not corrected because the cost of doing so is too expensive.