This discussion really applies to any brand on any camera.
Lens skew is all too common and yet this sample of the Sigma 35/1.4 showed itself to be an outstanding peformer on most all other field shots (at least at closer range). But given a ~infinity focus shot with the subject at a uniform distance, lens skew stands out like a sore thumb.
Sigma is not being singled out here, but in this case I was able to produce an unusually instructive case with this otherwise outstanding lens. Regrettably because it is a loaner I am not at liberty to send it to Sigma to see if it deemed “within spec” or not.