One theoretical feature that makes item response theory (IRT) models those of choice for many psychometric data analysts is parameter invariance, the equality of item and examinee parameters from different examinee populations or measurement conditions. In this article, using the well-known fact that item and examinee parameters are identical only up to a set of linear transformations specific to the functional form of a given IRT model, violations of these transformations for unidimensional IRT models are investigated using analytical, numerical, and visual tools. Because item parameter drift (IPD) constitutes a lack of invariance (LOI) at the individual item level or item set level, the magnitudes and effects of IPD on examinee response probabilities and true scores are algebraically derived and connected to empirical results from a recent simulation study. Thus, this article facilitates a deeper understanding of the exact statistical formulation of parameter invariance as a fundamental property of latent variable measurement and explicates some practical consequences of LOI for decision making.