Regarding Peter's Astra, seen that before, that protective cover holds road salts etc, in addition the part you can see normally "looks" ok, it is the part you cannot see that corrodes.

Another one I had was on a MK2 Mondeo with the rear brake pressure reducing valves that connect into the rear flexies blow apart on test.

I hope Dave's is not a trick question , from a testers manual? I am not a tester but would replace that and show the customer why bearing in mind its going to be on the road another year