Stephen Fry was never able to convincingly fake hitting someone, so often he genuinely hit Hugh Laurie while filming the sketches.
In the third and fourth seasons, when Stephen Fry would make a bizarre cocktail over the end credits, all the mishaps that would frequently occur were kept in the final cut. Also, a second cocktail shaker was usually visible on the table containing the actual drink that Hugh Laurie (and in season 4, the guests) would consume.