The shutdown frequency and duration for mill concentrators is generally dictated by the primary mill reline, and this is especially true for grinding circuits with SAG mills. As a result, mill liners are routinely inspected to understand the current wear performance and forecast the service life with respect to the scheduled shutdown dates. These inspections are now typically performed using 3D laser scanners, which generate comprehensive models from millions of data points collected from the exposed liners and mill charge in situ. 3D scanning is a major improvement over traditional methods like manual ultrasonic measurements. However, it has not consistently reduced premature liner failures or emergency relines. A key reason for this is a poor understanding of liner wear fundamentals from mill operators and plant metallurgists. This paper revisits the fundamental principles of liner wear, its application to wear monitoring and forecasting. It highlights common pitfalls and misinterpretations in current practices, providing guidance to help operators avoid costly liner-related failures and improve the reliability of shutdown planning.