This happens very often during gameplay. Not mission related that i can tell.
When in combat and trying to reload the heavy stubber. If you initiate this process whilst holding Right click down the stubber tends to just get reloaded over and over again, the animation appears to carry on for as long as you hold right click and the gun never actually reloads until right click is let go.
I have not been able to recreate this is the psykhaniam so i presume this is related to attempting to reload whilst being hit in close combat.