It was definitely a mishap. Both gun shots were also faulty in the first preview, as in Eva fired and made the movement of shooting, but the sound effect was delayed.
The sound effect is done directly by the gun that Kim (Eva) holds, it shoots blanks. It makes sense when you need to sync her movement with such an important sound effect.
When the prop on stage doesn't work, there's someone in the wings with a backup prop (or something similar that makes the sound effect) ready to cover the mishap.
I guess when you saw it, both the prop and the back up did not work, which really sucks.