Wondering if any Canon DSLR users out there in Cyberspace have run into this peculiar problem. Seems to happen after the camera has been in use off and on for a few hours - wedding photography. Anyway, what happens is ... I'm strictly using the center focus point, focus on my subject, shutter release depressed halfway, then I recompose the shot. Shutter release still being held halfway - no change in pressure being applied, and when I depress the shutter release all the way down there is a VERY noticable lag before the shutter activates and when it does the center focus point re-focuses on wherever it's now aimed a split second before shutter activation. All this after recomposing my shot. Now my subject is of course out of focus. This problem will almost certainly have to be investigated and repaired by Canon, but I'm trying to find out if anyone has ever had a similar problem and what the outcome was.
Thanks to all who respond.
Ps ... in all cases I've had a 550 EX flash attached but not necessarily turned on.