Virtual and augmented reality head-mounted displays (HMDs) are currently heavily relying on spatially tracked input devices (STID) for interaction. These STIDs are all prone to the phenomenon that a discrete input (e.g. button press) will disturb the position of the tracker, resulting in a different selection point during ray-cast interaction (Heisenberg Effect of Spatial Interaction). Besides the knowledge of its existence, there is currently a lack of a deeper understanding of its severity, structure and impact on throughput and angular error during a selection task. In this work, we present a formal evaluation of the Heisenberg effect and the impact of body posture, arm position and STID degrees of freedom on its severity. In a Fitt's Law inspired user study (N=16), we found that the Heisenberg effect is responsible for 30.45% of the overall errors occurring during a pointing task, but can be reduced by 25.4% using a correction function.