Either the trigger moved forward, not engaging the sear or the slide moved sufficiently to engage it, either way the gun is perfectly safe and can be beaten to death without discharging a round.
The 3 safeties in the Glock platform are designed to disengage progressively as you pull the trigger, you can use the gun as a hammer with a round in the chamber and it won't discharge unless the trigger is pulled, that is if all the safeties haven't been tampered with, specially the plunger, which will not allow the striker to hit the primer
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