This code is subtly wrong:

func f() (err os.Error) {
  v, err := g();
  if err != nil {
    return;
  }
  if v {
    v, err := h();
    if err != nil {
      return;
    }
  }
}

The := in the if statement causes a new err variable that shadows the 
return parameter.

Maybe doing this should be an error. Maybe return parameters should be 
special so that := doesn't ever shadow them. (I like the latter.)