As part of TSL’s ongoing investigation into the relationship between surface flatness and transmitted wavefront error (TWE), we spotted the TWE dataset below whose anomalously high peak has a distinct touch of the Halloween about it. Comparison with maps of surface flatness shows that highly localized wavefront distortions such as this can sometimes be caused by surface defects but are sometimes the result of internal defects in the viewport optic. The investigation is ongoing, but (spoiler alert) the magnitude of the wavefront distortion is usually significantly less than the magnitude of the surface flatness. Both are commonly expressed in terms of testing wavelength (λ). Happy Halloween!