This is my favorite.

>1979: Afghanistan — The Soviets invade Afghanistan. The CIA immediately begins supplying arms to any faction willing to fight the occupying Soviets. Such indiscriminate arming means that when the Soviets leave Afghanistan, civil war will erupt. Also, fanatical Muslim extremists now possess state-of-the-art weaponry. One of these is Sheik Abdel Rahman, who will become involved in the World Trade Center bombing in New York.

The US literally armed its own enemies.