How a ballpoint pen saved the Apollo 11 space mission:

Neil Armstrong, the first man on the Moon, and Buzz Aldrin, his fellow astronaut, accidentally snapped off the switch of a circuit breaker, and found they could not take off without it.

Aldrin then jammed a ballpoint pen into the hole where the switch had been, allowing the astronauts' lunar module Eagle to leave the surface of the Moon.

(Via Gravity Lens.)