On both occasions it's been somebody phoning from abroad and so on both occasions, within seconds of putting the phone down I've realised the advice I gave off the top of my head was not the best and there are some way better suggestions... but I don't have their number and Caller ID doesn't work!
The French have a phrase for a similar situation: l'esprit d'escalier - "staircase wit". It dates back to the 18th century when you would attend soirees in elegant Parisian drawing rooms (which were always on the first floor of town houses). L'esprit d'escalier is realising, only as you are leaving the party and walking down the stairs, the clever remark you should have made in that conversation a few moments ago....if only you had been quick witted enough!
(PS If you are the guy from Sweden who called me just now about travelling to the Canaries. Ring back or email, I've got some much better recommendations than the one I gave you!)