Super-famous and very good-looking authors Karl Wiegers and Joy Beatty did a fabulous job of describing this problem in a section titled "When Bad Requirements Happen to Good People" from their amazing book Software Requirements, Third Edition (Microsoft Press, 2013), reprinted here with permission:
When Bad Requirements Happen to Good People
The major consequence of requirements problems is rework—doing again something that you thought was already done—late in development or after release. Rework often consumes 30 to 50 percent of your total development cost, and requirements errors can account for 70 to 85 percent of the rework cost. Some rework does add value and improves the product, but excessive rework is wasteful and frustrating. Imagine how different your life would be if you could cut the rework effort in half! Your team members could build better products faster and perhaps even go home on time. Creating better requirements is an investment, not just a cost.
It can cost far more to correct a defect that’s found late in the project than to fix it shortly after its creation. Suppose it costs $1 (on a relative scale) to find and fix a requirement defect while you’re still working on the requirements. If you discover that error during design instead, you have to pay the $1 to fix the requirement error, plus another $2 or $3 to redo the design that was based on the incorrect requirement. Suppose, though, that no one finds the error until a user calls with a problem. Depending on the type of system, the cost to correct a requirement defect found in operation can be $100 or more on this relative scale. One of my consulting clients determined that they spent an average of $200 of labor effort to find and fix a defect in their information systems using the quality technique of software inspection, a type of peer review. In contrast, they spent an average of $4,200 to fix a single defect reported by the user, an amplification factor of 21. Preventing requirements errors and catching them early clearly has a huge leveraging effect on reducing rework.
Shortcomings in requirements practices pose many risks to project success, where success means delivering a product that satisfies the user’s functional and quality expectations at the agreed-upon cost and schedule.