From Canon's website ...
Firmware Version 1.1.0 incorporates the following fixes.
1. Corrects a phenomenon that in images captured by continuous shooting, and under certain conditions, barely noticeable traces of the immediately preceding frame may be visible.
Firmware Version 1.1.0 is for cameras with firmware up to version 1.0.9. If the camera's firmware is already version 1.1.0, it is not necessary to update the firmware.