Gemini 8

Gemini 8 (officially Gemini VIII) was the sixth manned spaceflight in NASA's Gemini program. The mission conducted the first docking of two spacecraft in orbit, but suffered the first critical in-space system failure of a U.S. spacecraft which threatened the lives of the astronauts and required immediate abort of the mission. The crew was returned to Earth safely. The only other time this happened was on the flight of Apollo 13.