This contribution investigates the influence of phase shift on the measured displacement error in interferometers based on quadrature detection. This error was experimentally investigated using a two-detector homodyne quadrature laser interferometer (HQLI) with two orthogonally polarized signals. Here, the phase shift can be continuously varied by rotating a wave plate. However, the rotation of the wave plate also produces unequal signal amplitudes and different zero offsets, both of which can be corrected with an appropriate signal processing. The measured phase-shift error perfectly agrees with the theoretically determined phase-shift error region. This error is systematic, periodic and severely asymmetrical around the nominal displacement value. For the robust realization of a HQLI, a slight deviation from the aligned angle of the wave plate should not shift the phase significantly from the ideal 90°. This may pose a problem if an additional phase shift originates from the polarization-sensitive light reflections, such as the reflection at the nonpolarizing beam splitter.