i was playing around with a friend's 6D at a NYE party. it was coupled with a 50/1.4, and a joy to use.
at one point, i was trying to take burst shots of some party silliness with a fast shutter to capture people jumping in the air. i noticed that the shutter wasnt rolling through completely - ie in a burst of 5 exposures, the 2nd or 3rd pic would have one half of the image darkened.
everything was lit with ambient light and i wasnt using a flash, so i dont know if this is something to do with the 1/180 sync speed. just found it strange that it the shutter wouldnt roll through fast enough across multiple exposures.