It happens often. Sometimes it works fine, you try and drop a weapon for a friend or make temporary space in your own inventory and it works but most of the time after dragging it to the vicinity area it just disappears, no trace. This is horrible when finding very valuable items like med kits or when your entire backpack with all your sweet loot just vanishes.

Also when exiting the game with a backpack and a weapon attached to you, upon re-entering everything is gone, you only have the items in your jacket and pants.