It was on this date in 1967 that astronauts Gus Grissom, Ed White, and Roger Chaffe were killed in a test of the Apollo space capsule in the lead-up to what would have been Apollo 1.
In retrospect, it's now clear that NASA was proceeding too fast into unknown territory. With just under 3 years left to fulfill President Kennedy's pledge to go to the Moon by decade's end, there was immense pressure on NASA to get an Apollo mission off the ground. Additionally, the Apollo space capsule was by far the most complicated ever constructed to that point. The result: corners were cut, many last-minute changes were made to the craft, and the crew was very nervous about the mission. Grissom, one of the Mercury 7, was especially vocal in his concerns.
During a routine plug-out test, where the ability of the craft to run off its own power was checked, a fire broke out at a point that was never determined. The fire, pure oxygen atmosphere, flammable materials inside the cabin, and inward-opening escape hatch held in place by a pressurized cabin all combined to doom the astronauts. The following investigation and changes to the craft design it demanded set back the Apollo program by over a year.
Ironically, even with this 1-year delay (which was far longer than the delay that would have been created by fixing the problems before the first test), we still fulfilled Kennedy's pledge.