There was some focus hunting when shooting birds in dense foliage with backlighting
Here's a shot that shows the bird and surround, a tough job for any AF system. This was with phase AF, and I haven't done an AFMA yet (need to find a deserted football field for that!). Might be a little back-focused, I think.
Shot is with the 1D X and 600mm f/4L
IS II + 2xIII, 1/320 s, f/8, ISO 4000. Original and a 100% crop to show detail, which is quite good, IMO.