Several causes of instabilities in the interaction of users with actively controlled haptic devices, notably sampling effects, sensor quantization, and hardware nonlinearities, have been identified and analyzed in prior work. However, certain instabilities may occur that cannot be attributed to these hardware limitations. The problem becomes acute when rendering passive virtual environments which require compensation of the intrinsic device dynamics. We show that passivity, the standard robust stability criteria for haptic interface systems, cannot be maintained in certain circumstances due to Bode gain-phase integral constraints. In an experimental study, we demonstrate that some performance can be recovered by relaxing passivity requirements while maintaining coupled stability. To test stability, we identify frequency domain models of the user interaction, and check the Nyquist stability criterion rather than passivity.