Vague requirements as a cause of software faults

Customers can use the VSR EasyFinance module to calculate various vehicle-financing options. The interest rate the system uses is stored in a table, although the purchase of vehicles involved in promotions and special offers can be subject to differing interest rates.

VSR-II is to include the following additional requirement:

REQ: If the customer agrees to and passes an online credit check, the EasyFinance module applies an interest rate from a special bonus interest rate table.

The author of this requirement unfortunately forgot to clarify that a reduction in the interest rate is not permissible for vehicles sold as part of a special offer. This resulted in this special case not being tested in the first release. In turn, this meant that customers were offered low interest rates online but were charged higher rates when billed, resulting in complaints.