The infamous "Snake Charmer" puzzle on the British version (November 1994), in which the partly-revealed animation appeared to show Mr. Chips engaging in an activity associated with 14-year-old boys. Cue everyone in the studio being reduced to fits of laughter.
It really didn't help that the squares that were picked just happened to be the ones revealing Mr. Chips' rather unfortunately-shaped head and the head of the snake (which gave rise to the alternative thought that Mr. Chips might be jacking off with the snake).
