The Canon 5D Mark III is a beast of a digital camera. At $3,500 customers rightly expect a high level of precision and perfection. The idea alone of some sort of fundamental hardware flaw would be enough to destroy the pristine image that the device is supposed to represent. So you can just imagine how damaging it must have been for Canon to come out and admit that there is a problem with the design of the camera.
Digital photo enthusiasts have been posting numerous videos about the problem, which makes the 5D Mark III's top LCD panel leak light into the body of the camera in extremely dark environments. This throws off the auto-exposure settings and in effect creates pictures that are completely unusable. At first, Canon remained mum on the issue, but after it circulated wildly around the professional photography community, the company has decided to issue a product advisory.
The advisory reads, "In extremely dark environments, if the LCD panel illuminates, the displayed exposure value may change as a result of the AE sensor's detection of light from the LCD panel." The problem with this is that it is in the hardware itself, so Canon can't simply release a firmware update. Its impact is minimal, since it only affects very dark situations when users have the backlight on, so it will be interesting to see if Canon actually takes the time to go back into the manufacturing side of things to fix this.