It sort of emulates MPEG2 macroblock errors where the decoder would give up on a row of macroblocks (leaving previous image data unchanged) and restart on the n

It sort of emulates MPEG2 macroblock errors where the decoder would give up on a row of macroblocks (leaving previous image data unchanged) and restart on the next line. In that case the glitches often started part-way along a line with a badly errored square macroblock in magenta or green.