Canon has issued a service advisory warning EOS 5D users that a problem with adhesive could result in the main mirror detaching. The camera’s available servicing period comes to an end this upcoming September, making it important that users get the problem fixed soon if it has occurred. The issue itself stems from the adhesive potentially deteriorating over time, causing the mirror to possibly come loose and obviously making the camera essentially useless (if you like to the use the viewfinder, that is). There are pics showing the potential issue after the jump.

You can see in the image below where the potential issue lies, and the solution Canon has to fix it. You’ll need to contact the Customer Support Center if your EOS 5D is showing the same issue.
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https://www.slashgear.com/canon-warns-eos-5d-dslr-main-mirror-could-detach-12383269/