Autofocus systems can be fooled

The idea of having to shoot a wedding without autofocus would not be an appealing prospect. Autofocus allows you to respond to situations incredibly quickly and is stunningly accurate. Modern cameras and lenses are designed specifically for autofocus - the throw on lenses (the distance the focal plane moves in response to adjustments to the focusing ring) is not suitable for manual focus. Autofocus is more accurate than my eyesight, even though I've replaced the Ee-A focusing screen in my Canon EOS 5D with the Ee-S variant (only suitable for lenses faster than f/2.8 due to reduced light reaching the viewfinder the Ee-S delivers - to quote Canon - "a steeper parabola of focus to make the image pop in and out of focus more vividly"). Autofocusing systems do, however, occasionally struggle. In the photograph above (wedding photography in Sandwich, Kent) the camera has focused on the wall behind the subjects, despite the fact that I focused upon the line of contrast between the bride's face and her hair, and the camera confirmed locking focus on this point (I only ever have one autofocusing point active on the camera - for the above shot the central one). I've got away with it in this case as shooting at f/8.0 has given me sufficient depth-of-field (DOF) [this photo has been significantly cropped, I was standing much further away than it appears giving a greater DOF which extends 1/3 in front of the focal point, 2/3 behind]. I've looked through a lot of other wedding photographer's books and online photos recently and noticed that this is a common occurence. The solution? If there is a tempting target for your autofocus system sitting just behind your subjects, focus on something else at the same distance - in the case above I'd be tempted by the border between the white and blue dresses. I'd be interested to hear your thoughts.