In this communication, an ultrathin transmissive half-wave plate (HWP) based on a bi-layered zigzag metasurface operating at the lower frequency edge of the terahertz (THz) spectrum is numerically and experimentally studied. The thickness of HWP is only 100 mu m and less than lambda/20 at the operation frequency, and it achieves an amplitude transmission efficiency over 90% and a cross-polarization discrimination around 30 dB within a fractional bandwidth near 9%. A detailed analysis of the device robustness with respect to layer misalignments is carried out by designing and fabricating two additional devices with the maximum possible shift between layers along both transverse directions. The results show that the device is extremely robust relative to a misalignment along x and exhibits a frequency shift with misalignments along y, while maintaining in all cases an excellent performance as a HWP. The communication ends with a final study to ascertain a physical mechanism that explains the robustness of the device in regard to misalignments. These results complement and extend the reach of metasurfaces in the emerging THz band.