All Canon DSLR cameras suffer from this problem, except the 5Dmk3.
Imagine looking out your window through horizontal blinds, watching someone wearing black and white striped clothing walking by.
One moment they may appear to be wearing completely black, then a second later they might appear completely white, or some ugly jagged diagonal pattern.
This can be controlled with an optical low pass filter (OLPF), effectively reducing the resolution to a level lower than the sampling interval of the sensor (so our friend with the striped clothes would appear to be wearing grey).
A company called Mosaic Engineering sells filters for the 5Dmk2 and 7D, and are also developing products for the 550D/600D and 60D. These must be inserted into the body of the camera directly over the sensor, if you take still photos with the filter in there the image quality will be reduced.
Morgan.