I don't think I've ever talked to Terry much directly (except one memorable moment at the first Irish DWCon when I was on my way to talk to Jack Cohen and continue a conversation from the day before, Terry was between us and I fumbled up my words trying to get past with something like an 'excuse me Mr, em I mea, Sir, em Terry, Sir Practchett, em, sorry!... ah Jack, there you are!'). I have since very calmly nodded and smiled and not attempted to used any title when he's nearby, and no longer get nervous if we're in the same conversation since I've already had my embarassing fan moment
My little brother has a good first moment with him though. Our first DWCon in 2006, all were warned at the opening ceremony about not bothering Terry when he had his 'other' badge on (Silas T. Firefly). My little brother at some point spotted Terry in the bar wearing his normal badge, went up as if to shake his hand saying 'Oh, Mr Firefly, I'm such a big fan of your work' but then paused, looked at the badge and then mock apologised 'So sorry, I thought you were someone else'. And simply wandered off nonchalantly.