On 27th 1967 astronauts Ed White, Roger Chafee and Virgil "Gus" Grissom were killed during a pre-launch test for the Apollo I mission at then Cape Kennedy (now Cape Canaveral) in Florida.
During the test a fire was reported in the cockpit of the Command Module. Although it was only brief, before the module ruptured, it lasted long enough to kill all three. Flights were suspended for 20 months and a wide range of flaws were found which contributed to the deaths.
This accident is the start of a bad week for NASA, as the Challenger and Columbia disasters happened within the same time frame, although in different years.
Image: By NASA (Great Images in NASA Description) [Public domain], via Wikimedia Commons (commons.wikimedia.org/wiki/File:Apollo_1’s_Command_Module_-_GPN-2003-00057.jpg)