TwoShoes wrote in post #12788061
Some nice shots on the last page!
My 1D IV is currently heading to Canon. I took it into the local company I purchased it from and after a few HOURS of testing the problem replicated itself consistently. You could take 10 frames of the same shot in either AF or MF and randomly 2-3frames would be OOF. The lens and camera would basically just stop communicating and you'd get no lens data in the EXIF on the OOF shots. Fortunately this happened over a range of lenses and Canon will hopefully take care of it.
Sad to see such a thing happen with such a beautiful camera.
My OOF percentages weren't that high, but I did experience them. When i first moved up to the Mk IV I spent a lot of time learning the best AF setting combinations for different situations, especially in SERVO. After 11 months, I had a few ERR 20 codes when shooting vertically. It seemed the mirror assembly would stay in locked up position, then drop, then clear, and all was good. The ERR 20 code showed up when it wouldn't clear itself. Off/On took care of it. It was random for awhile, then became consistent in vertical orientation.
I sent it in for warranty repair identifying the ERR 20 code and mirror assembly as a possible culprit and received it back a week later and they had replaced the AF system as the problem. Have been shooting indoor swimming competitions in the last two weeks--slower shutter speeds, humidity, etc., and I can stuff 8-10 fps into my CF cards all night and my SERVO tracking and focus accuracy is never a concern now. I'm sure some of my early problems were operator and settings errors, but I now also wonder if there wasn't some kind of inherent disharmony between electronic and precision mechanical systems.
It's shooting so sweet right now i almost don't want to upgrade to the 1Dx 
