Lines on my H.264 video export
Ok, now that I'm home I think I see what you are noticing.  That is just an error with the block transitioning from black to white.  You can either slow down the fade or increase the keyframe frequency.  H.264 works by storing only certain frames and then storing the change in-between.  It breaks the image in to sections to analyse and so you can see if you go frame by frame that it is having a hard time figuring out what the change is around the edge between black and white.

Either decreasing the rate of change or increasing the frequency of keyframes should give it a better idea of what is happening.  You might also try 2 pass VBR if you were only doing one pass or CBR prior as the analysis pass should help clean up artifacts like that by directing more bitrate to harder areas to compress.