Just months before the first moon landing, Neil Armstrong was squeezing in a few more practice landings. In an altogether disconcerting turn of events, the lunar lander exploded during practice.
Between our stronger gravity and thick atmosphere, it's hard to practice landing on the moon by flying on Earth. The best the test flight engineering team came up with was a craft where they could vertically take off to a certain altitude, then switch to a moon-scenario mode where the engines produced less lift. In this altered condition, the craft would slowly sink under one-sixth of its mass, matching the one-sixth gravity of the moon. The mock-up worked well, providing both Neil Armstrong and his backup Pete Conrad with an opportunity to practice soft-landings under lunar free-fall conditions.
In May 1969, the Apollo 11 crew were undergoing the final set of practice exercises and checklists leading up to the moon landing in July. The May 6th lunar landing rehearsal at Ellington Air Force Base was Armstrong's 21st practice flight, and the 34th for the vehicle. Shortly after initiating the lunar free-fall mode, the practice flight went horribly wrong when the craft rolled and became unresponsive. Armstrong ejected, flying free of the craft just moments before it crashed and exploded. To the surprise of his coworkers, Armstrong continued to work for the remainder of the day.
Although this seems like a massive "...whoops" moment, the post-accident report discovered the problem was with the fuel, not the craft. Armstrong went on to land the lunar module on the moon far softer than expected by the engineers, and credited his achievement to the high-quality practice provided by the training craft.