[en] Theoretical and experimental fully differential cross sections are presented for electron-impact ionization of molecular hydrogen in a plane perpendicular to the incident beam direction. The experimental data exhibit a maximum for 1-eV electrons detected 180 deg. apart and a minimum for 10-eV electrons. We investigate the different physical effects which cause back-to-back scattering and demonstrate that, over the energy range from 10 to 1 eV, a direct transition is observed from a region where Wannier threshold physics is essentially unimportant to where it completely dominates.