Canon has published a product advisory notice acknowledging the existence of a 'light leak' issue on the EOS 5D Mark III. Users of the camera have reported that in dark conditions, switching on the top-LCD's back-illumination or shining a light on it alters the camera's metering, suggesting light is leaking into the body and reaching the metering sensor.
The company says it is investigating the problem and possible countermeasures, and will make further announcements once a decision has been made on resolving the issue. The full advisory note is on the Canon USA website. Petapixel reported on the issue and has linked to user videos which illustrate the problem.