http://www.usa.canon.com/consumer/controller?act=PgC
omSmModDisplayAct&fcategoryid=139&modelid=19356&keyc
ode=2112&id=60427

"In images captured by continuous shooting, and under certain conditions,
barely noticeable traces of the immediately preceding frame may be visible.
This phenomenon is not noticeable in an image with optimal exposure. The
phenomenon may become more noticeable if a retouching process such as level
compensation is applied to emphasize the image."