A number of users have reported experiencing “light leaks” when they used their Canon EOS 5D Mark III to shoot in low light. Canon is aware of the problem and has described it like so:
Concerning the EOS 5D Mark III digital SLR camera, when the LCD panel illuminates in extremely dark environments, the displayed exposure value may change.
Canon UK suspended further shipment of the 5D Mark III, while Canon USA promised to look into the matter.
Canon USA concluded their investigation today, posting:
Based on the results of extensive testing this change in exposure value will not noticeably affect the captured image.
So basically, Canon’s saying everything’s all good with the issue since your images will turn out fine even though the exposure value might jump around a bit. However, you can have your camera checked out by Canon if you want to make sure it’s working fine. The manufacturer will be offering the inspection service for free starting in mid-May. You can reference their announcement for contact information.
Canon also requested that camera owners register their EOS 5D Mark III to receive further service updates.