Gemini VI-A
Originally scheduled to rendezvous with an Agena target module, Gemini VI had to go through a drastically modified flight plan when the Agena rocket blew up on its way to orbit. Engineers quickly assembled a change of plan that called for the Gemini 7 flight to be slight altered and equipped with some additional hardware that allowed the Gemini VI to first locate the Gemini VII and then dock with it.
The new mission plan called for Gemini VII to take off first and then several days later the Gemini VI (now called Gemini VI-A to designate the mission change) would lift off from Kennedy. Gemini VI-A would be manned by command pilot Astronaut Walter M. Schirra, Jr., and pilot Astronaut Thomas P. Stafford.

NASA PHOTO: NASA attempted to launch the Gemini 6 at 9:54 a.m., December 12, 1965. However, seconds after ignition the first stage engine of the Gemini Launch Vehicle 6 shut down due to a faulty release of a liftoff umbilical plug.
The new mission would require a quick 1 week turnaround of the pad after launch, which would be a big accomplishment. With VII already circling the Earth, VI-A was prepped and ready to go, but the launch had to be aborted. The Titan II ignited for just a moment, then shut down and settled back down on its launch attachments.
Kenneth Hecht, chief of the Gemini Escape, Landing, and Recovery Office and long-time ejection seat specialist, was surprised when the crew did not eject, as they should have if ground rules had been strictly followed. Although the clock had begun to tick indicating there was a liftoff, both Schirra and Stafford agreed that they felt no sense of upward lift. Had the fully fueled Titan moved even a few centimeters upward and the engines suddenly cut off the momentum of the fuel, which was about 150 tons that was held in a relatively thin shell, would have most likely put so much stress on the tanks that they would have ruptured.
A few tense moments followed. Schirra waited it out, holding off pulling the abort handles that would have sent the men catapulting out of the capsule on notoriously unreliable ejection seats. The booster was secured and Schirra saved the mission and the launch three days later went perfectly.
Wally Schirra's decision not to pull the D-ring that would have blasted the 2 astronauts from inside their capsule was fraught with speculation. Even astronauts who were mentally prepared for the test firing of the escape hatches suffered serious injuries. It was not a procedure to take lightly. Schirra remarked:
"If that booster was about to blow. . . if we really had a liftoff and settled back on the pad, there was no choice. It's . . . death or the ejection seat."
Mission VI-A was changed from a docking procedure with the Agena module to a rendezvous with VII capsule. What this meant is that VI-A had to approach VII to within about 120' and maintain that distance without it varying. It was felt that if this could be done successfully, then it would be possible to fine-tune the ships so that could actually dock with each other.
During the rendezvous, the 2 ships remained in close proximity and performed delicate maneuvering moving the ships closer to each other and then backing away. At one point they even brought the ships together nose-to-nose and remained in this flying configuration for several orbits without any problem.
Ten days before Christmas in 1965, the astronauts aboard Gemini 6 performed a rendition of the holiday tune "Jingle Bells" using a harmonica and bells.
That concept would be further developed in future missions.