What ruined it: A poker game.
While Washington's heroic crossing of the Delaware may be firmly implanted into the American psyche, the truth is actually a whole lot stupider than what we were taught in history class. It turns out a loyalist spy named Moses Doan observed Washington along the Delaware on Christmas Eve, along with what he described as "a boatload of soldiers." He immediately notified Col. Johann Rall of the Hessians about the impending attack but was turned away because the colonel was engaged in a heated game of chess -- or, depending on who you ask, a poker game.
Since the colonel had no wish to be disturbed, Doan left him with a note that read: "Washington is coming on you down the river, he will be here afore long." Rall, still deeply engrossed in his game, stuffed the note into his pocket without giving it a thought.
"I had a pair of threes. You never back down on a pair of threes."