The concept of the Concentration '73 bonus round wasn't difficult in and of itself IMO, but when the showrunners insisted on solutions like "JIMMY STEWART WAS A FAMOUS ACTOR WHO STARRED IN IT'S A WONDERFUL LIFE" and "MY VERY EXCELLENT MOTHER JUST SERVED US NINE PIES," it definitely became one of the hardest.
I'd say Carmen Sandiego once they increased the goal to 8 correct answers ranks up there, especially if a contestant was unlucky enough to get the Africa map. And even though it was thoroughly discussed on the board recently, Nick Arcade.