There are actually multiple potential sources of error here. [EDIT for disambiguation: I mean error in reconstructing the image at full RGB resolution, not erro

There are actually multiple potential sources of error here. [EDIT for disambiguation: I mean error in reconstructing the image at full RGB resolution, not error in what anyone has been saying in comments here.] 1. The antialiasing filter. This behaves very much like a defocusing error. 2. Demosaicing. Exactly what this does depends on the algorithms in the camera. It doesn't necessarily look much like defocus. (For instance, on some cameras I've worked with -- not consumer ones, FWIW -- you get demosaicing artefacts that produce a sort of tartan effect.) If the antialiasing filter is strong enough that it destroys all information in the image below the Nyquist limit for the R and B photosites, then demosaicing can in principle be perfect -- i.e., lose no information that wasn't already lost by the antialiasing filter. But (a) that ceases to be true in the presence of noise in the sensor, and of course there usually is some, and (b) an antialiasing filter that strong throws away information because then you're not really using all your green photosites. So in practice, sometimes you'll get ugly high-frequency demosaicing artefacts. Presumably designing a good demosaicing algorithm is largely about making this happen as seldom as possible for real-world images.