Undergraduate grade point average (GPA) is a commonly employed measure in educational research, serving as a criterion or as a predictor depending on the research question. Over the decades, researchers have used a variety of reliability coefficients to estimate the reliability of undergraduate GPA, which suggests that there has been no consensus on the most appropriate model. This paper reviews the assumptions of different reliability models and examines the effect of violating these assumptions on reliability estimates of GPA. Using longitudinal semester GPA data for 62,122 students from 26 four-year institutions, the reliability estimates for semester, annual, and fourth-year cumulative GPA ranged between 0.60-0.65, 0.75-0.79, and 0.89-0.92, respectively. Depending on the measure, up to eight different reliability coefficients were estimated. In general, different estimates resulted in minor differences even when the assumptions of the underlying models are not met; however, larger differences were observed for the fourth-year cumulative GPA analyses.