One early spaceflight had a complete rocket engine failure after a “flight” of only a few feet up from the pad. As the main rocket stalled, then fell back and began burning, the capsule escape rockets fired (by the default failure of their own safety circuits), and blasted the capsule completely clear of the burning area. NASA signed off on the “capsule escape rocket test flight” ... that had not been scheduled yet!
A few other Mercury escape rocket tests were not so successful.
https://www.youtube.com/watch?v=p0w_xyePC_0