Just above the order—disorder critical temperature (Tc) in Cu3Au several unusual effects have been reported: (1) The ratio of diffuse intensity at the 003 position to that at the 001 decreases markedly. (2) There is a sudden increase in the total diffuse X-ray scattering due to mean-square displacements. (3) Phonon groups broaden. (4) Optical modes appear. Additional details of these effects are described here. Also it is shown that: (5) there are no significant changes in the phonon dispersion curves; and (6) there is a marked increase in (quasi-static) anharmonic interactions (which were observed in the X-ray diffraction pattern). All of these unusual effects are interpreted as arising from long-lived fluctuations above Tc, like the low temperature ordered phase. The results suggest these fluctuations contain a high density of thick antiphase boundary.