Poor design or product implementation.
A bad design may not be manufacturable or live up to product claims.
What to do about it!
Develop precise engineering requirements, based on the total product requirements (from a complete Market Requirements Document), and then review and verify the design throughout the development process. As before -reviews should not become a commodity activity, but be a considered activity at points in the process that provide pertinent product and project visibility to the various stakeholders.
Poor testing strategy.
A product that can't easily be tested can't easily be verified, validated or easily repaired and may even be discovered to be inadequate after it's in the marketplace.
What to do about it!
Design a test methodology during the product definition and specification stages, not as an after-thought. Take into account all phases of testing the product may face: verification, compliance, validation, assembly, QC, installation, user diagnostics and repair.