Walker's exclusion always intrigued me, and I think that may be the flat feet one mentioned, but I'm sure he says it's because he's in a reserved occupation, and it's sniggered at by Frazier who then stirs up trouble by asking the others what it is Joe is barred from the army for. I think they came out with lots of fanciful reasons and someone (Wilson?) lets out of the bag it was flat feet. Or something. I'm very hazy on this one, I can't picture the episode it was.
Unless I'm mixing more than one episode up, as I'm pretty sure of the scene he says reserved occupation, and it doesn't fit in with the rest of it. Unless he said it in more than one.