Surely I can't be the only one to do things like this.
Picked up the 50D this morning with the 400/5.6 and found that it wouldn't focus. I thought maybe I had somehow switched the back focus button off and tried the shutter half press. Nothing. I manually focused the lens to "ballpark range" and THEN it would autofocus just fine. Started getting concerned that maybe the lens is having issues, or the body might be bum or....
Then I realized that last night I had turned "Disable focus hunting" on.. oopsie. It works like a charm to keep the lens from going too far out of focus when shooting against a low contrast background (e.g. blue sky) so is really nice for birding, but it also means that when the lens is too out of focus it just won't attempt to move it at all.
Now I don't feel so smart.