I walked past Terry Prachett in the basement of the Forbidden Planet store when it was near Centre Point in London. He was sat down with two or three people around him,signing a book or something like that- not sure if it was something official or impromptu(since I didn't notice anything about him being there in the front of the shop).
Also I managed to get Jeremy Clarkson to comment on a loud shirt I was wearing many years ago,which was amusing.