This chapter investigates the most prevalent well integrity failures originating from the construction phase, emphasizing how design, material, and execution flaws can lead to long-term barrier degradation. Drawing from field statistics, forensic analyses, and post-abandonment studies, the chapter categorizes failures into mechanical, hydraulic, and chemical types—including poor cement bonding, casing collapse, micro-annulus formation, channeling, annular pressure buildup, and wellhead seal erosion. Each failure mode is traced to its root cause, such as improper centralization, inadequate mud removal, poor cement slurry design, casing wear, poor torque and drag modeling, and ineffective pressure testing. The chapter uses failure trees and fault-mode-effect analyses (FMEA) to illustrate how minor construction oversights can propagate into significant integrity breaches. Case studies demonstrate how substandard cement placement in high-deviation wells, under-designed casing strings in HPHT conditions, or failure to identify gas-cut cement can result in costly interventions, sustained casing pressure (SCP), and regulatory non-compliance. Attention is also given to failures associated with premature degradation from CO₂, H₂S, and brine exposure in early service life. Corrective recommendations are provided, linking failures to preventive measures such as risk-based cement design, load path modeling, and enhanced diagnostics during well construction. By uncovering the causality between early-phase errors and later-life failures, this chapter establishes a feedback loop for continuous improvement in well design and execution.