Canon has issued a service notice concerning its A650 IS series cameras. The company has detected that a number of A650 IS cameras have a light leak issue when used with the swivel screen opened.
Powershot A650 IS cameras with a serial number in which the fifth digit from left is a zero are affected (xxxx0xxxxx).
Even if your camera has such a serial number, it's possible that it has already been modified, and you can check that by opening the battery compartment.
You can see detailed instructions with pictures on DPreview, here.