EDIT: I changed the title to more accurately describe the issue. Seems to be a show specific issue rather than a Nat Geo Wild issue.
I've only watched 2 shows on N.G. Wild and both have done the same thing.
The best way I can describe it is if you have ever looked through a window when it's raining and the water distorts the background, that's what it's like. It comes in waves. It only effects part of the screen as to the "watery distortion" effect leaving the rest normal. It's not like pixelation where chucks of the image are missing/blocky, but it's more like the image is being folded over itself ever so slightly, if that makes sense. (Again, think of water running down a window.)
One show is The Dog Whisperer, the other is Leader of the Pack. I recorded 7 episodes (Marathon) of Leader of the Pack the other day and all 7 episodes did it. I am watching my first recorded episode of Dog Whisperer right now and it's doing it as well.
The effect is in the recorded data and reproducible 100% where the pixelation issue that crops up from time to time is 95% of the time not reproducible when rewound and watched again. Though it is from time to time in the recording as well.
I've never seen this on any other recording/station, just N.G. Wild.
Short of this issue, the HD quality is quite good!
I just noticed Leader of the Pack recorded a new episode just now, and it to has the same effect.