The buzzer sound for Daniel Radcliffe is, rather predictably, "Expelliarmus!", of Harry Potter fame. But due to licensing issues, the particular sound-byte used is not an actual excerpt from any of his Harry Potter films.
With his appearance here, Daniel Radcliffe became the youngest panelist on the show, at the age of 21.
So that the realistic effect of the magic trick involving Daniel Radcliffe would pass the "watershed" airing time conditions, a tagged close-up shot of him quite unharmed had to be inserted after the end credits.