Further to your enquiry we would like to inform you that we very recently (in April) have become aware of this and is now a known issue with the EOS 5D Mark III model. The AE sensor in the camera detects the light from the LCD panel when it is turned on and the exposure value will be altered. The change is not significant as it will be altered by approximately 1/3rd of a stop but can be noticeable. You can continue to use your 5D Mark III and the LCD screen can be turned off to receive the correct exposure.
The video above shows examples of what this 1/3 stop difference does for nighttime photographs. The issue definitely isn’t a huge one (don’t cancel your orders), but the 5D Mark III is a $3,500 camera and it’ll be interesting to see how Canon decides to deal with this flaw.