Posterous knows when you add an external link
Of course, for this to work I'd have to add http:// which on this occasion I neglected to do. Luckily the clever little Posterous interface reminded me, via a pop up, asking if I would like them to add it for me. I did and they did.
If you can guess and detect possible errors that your users might make and suggest possible solutions then try to incorporate that into your app.
