All three Apollo 1 astronauts were killed during a pre-launch test. In the months leading up to the disaster, there were several red flags that cemented the crew's fate.
“The first manned Apollo mission was scheduled for launch on 21 February 1967 at Cape Kennedy Launch Complex 34. However, the death of the prime crew in a command module fire during a practice session on 27 January 1967 put America’s lunar landing program on hold.”
The Apollo 1 Launchpad Fire: Remembering Grissom, White and Chaffee
“Test pilots can sense straightaway if they're working with a good vehicle or a bad one, and the Apollo 1 crew . . . knew almost immediately that they'd been assigned to a stinker. ”
Heat and Ashes: The Untold Story of the Apollo 1 Fire
“A great deal of progress had been made since the first manned spaceflight programs, Gemini and Mercury. But the space race with the USSR, combined with Kennedy's public goal of a moon landing, created a growing sense of urgency to advance the Apollo program at a breakneck pace. As a result, everyone on the Apollo team felt a personal responsibility to put in marathon time.”