With the Canon 5d Mk1 could happen, that the main mirror fells out from it’s frame. This is a known problem. In that case – but only after happens – Canon repairs this in lifetime warranty. Only the mirror, which means if the matte screen has been scratched, will be not replaced.
Today i have got a practically unused 5d, with 4042 shutter activations. I promised to the owner to check the mirror for a possible damaged. I hanged very-very carefull a nail of one finger between the glassplate and the frame. The mirror fell out immediately, without any resistance.
I am convinced now, that this mirror problem is caused not because the exposures, but definitely by the wrong quality of the glue and the aging.
Everybody should take the right conclusion for himself.