(Canon EOS 5D Mark III image courtesy of Canon U.S.A.)
After reports of a possible defect in the Canon EOS 5D Mark III professional dSLR circulated for a few weeks, Canon has issued a product advisory for the product, acknowledging the problem. In Canon's words:
The phenomenon described below has been confirmed when using the Canon EOS 5D Mark III Digital SLR Camera. Canon is now examining the countermeasures and once the countermeasures are decided, we will post the information on our Web site.
Phenomenon - 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 defect seems to indicate that light is somehow leaking through the top LCD panel area to the AE sensor, and therefore affecting the exposure value. Posts on Petapixel and Canon Watch include videos that highlight the defect. One shows how turning the LCD panel's backlight on in a dark environment (or with the lens cap on) alters the exposure value displayed. The other shows a similar effect as it occurs in bright light, when the LCD is shaded or exposed to the light.
Canon Watch also printed an email response that a 5D Mark III owner claims to have received from Canon that further confirms the issue and indicates that it is due to the AE sensor detecting light from the LCD panel.
Though many won't find the problem to be a big issue (turning off the LCD backlighting can be a simple solution), it doesn't seem like a problem that can be resolved with an easy fix like a firmware update. If it turns out that a physical manufacturing defect has to be repaired, the only way to do so would be to have owners send their products back for servicing, not something Canon would look forward to, obviously.