When your inventory is full and you have a drum mag in your gun,and a 30rnd magazine in your inventory (Drum mag takes more inventory space than 30rnd akm magazine) Then when you try to open a door you accidently press "Reload 30rnd akm magazine" Than it puts the 30rnd magazine into your gun and the drum mag disappears instead of falling into the ground.(Because it doesnt have enough space to go into inventory)