I haven't had this problem with my AF bodies ... yet, but I used to own a pair of the original 645 Supers.

I was just zippin' along shootin' up a storm one day ... after trying to hunt down the cause of my focus problems for a month or so ... when the film mag came off in my hand!

... dark slide out, film loaded and in the middle of a roll!

Come to find out the was an alignment problem with the body and film mags that was the cause of my focus problems. Fortunately there was a Koean gentleman who was the regional MAC authorized repair person and he fixed both bodies and all three mags for less than $100!
... he also installed a sync speed lock, a nice touch since the original Supers only locked the shutter dial into 'Auto' mode in their 'stock' configuration.