We've all read the various accounts of what allegedly happened, and heard what the wounds Mr. Whittington suffered are alleged to be. While I haven't gone TOO crazy with these articles, I was letting the info percolate in my head and I came to this ( to my thinking ) plausible scenario.
Cheney, Whittington and the rest of the party were out walking a line through the field, spread out several yards from each other, in a standard formation. Cheney is carrying his gun pointed to one side with a slight upward angle. His safety is off and his finger is in the trigger guard, but not necessarily on the trigger. He stumbles, grips the gun so as not to drop it, and it goes off, wounding Whittington.
I don't KNOW that this is what happened, of course. However, Occam's Razor works a bit better with this than it does with the whole "retrieving bird-sneaking up behind-wheeling around and blasting" line we've been told. Though I don't see how EITHER story makes Cheney look anything but incompetent with a gun.