Here's A quick video showing the (by now) well-documented "light leakage" issue with the Canon 5D Mark III. In short, the top LCD "leaks" light when strong light hits it, or when the backlight for the LCD is activated in dark lighting conditions. This only affects the auto exposure modes (Av/Tv/P/Auto, etc) and obviously not manual. However, since it affects the metering, your in-camera meter will jump around if this happens and you're shooting in manual mode. [youtube]http://www.youtube.com/watch?v=4kvUcoQhrmc[/youtube]
Here's a response from canon to someone on the dpreview forums who submitted a concern:
"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."
In my experience, it is more than 1/3 of a stop; it's more like 1 full stop. Canon issued a product advisory last week regarding the issue, so let's hope they can fix it. It seems like a hardware defect, so it's unlikely that it'll be able to be fixed through firmware, which means that owners will probably have to send their cameras in to get it fixed, if and when they find a fix. Not looking forward to being without the camera for a week or two, especially since once they announce a fix, the service centers are going to be slammed with incoming cameras.
Really, it's more of an annoyance than anything. Apparently it doesn't happen often when a lens is mounted and the camera is in normal use. For it to happen in "normal use" the lens cap would have to be on, you'd have to be looking through the viewfinder (blocking any incoming light via that path), the subject would probably need to be shaded, and you'd need to have some strong light hitting the top LCD panel....Or something like that, which equates to it not actually happening while shooting.