"It seems a new issue has popped up for the Canon 5dmk3. Multiple people are reporting that there is a light leak through the top LCD panel that directly affects metering. The easiest way to detect this issue is to put the body cap on the camera body as well as cover the viewfinder. Then place the camera in “P” mode at ISO800 and press the LCD backlight button."
I don't know if this is so much a disaster for Canon shooters. It doesn't sound like it affects every camera either, according to the comments on this page. For myself, if it is a problem I found with my camera, wouldn't it only be an INTERNAL one? It doesn't seem to be suggested here that the light meter is affected by external light sources other than the lcd backlight; and who's using that while making an exposure? If the camera's exposure was changing/fluctuating while the lens cap was on AND the viewfinder covered while I put the camera body into and out of a dark environment, I might be more concerned since it would suggest there is a real light leak in the body of the camera which lets in outside light apart from the lens and the viewfinder.