wikipedia posted:

The rocket only rose about 4 inches (10 cm) before settling back onto the pad. Alarms were immediately sounded at LC-5, but the Redstone didn't explode. Instead it merely sat in place, after which a strange sequence of events happened.[4][6][8]

Immediately after the Redstone's engine shut down, the Mercury capsule's escape rocket jettisoned itself, leaving the capsule attached to the Redstone booster. The escape rocket rose to an altitude of 4,000 feet (1,200 m) and landed about 400 yards (370 m) away. Three seconds after the escape rocket fired, the capsule deployed its drogue parachute; it then deployed the main and reserve parachutes, ejecting the radio antenna fairing in the process.[4][6][8]

In the end, all that had been launched was the escape rocket. However, the fully fueled and powered-up Redstone was now sitting on LC-5 with nothing securing it to the pad.