http://www.goldhawkinteractive.com/forums/showthread.php/7510-v19-Stable-Hotfix-Soldier-Equip-Screen-Can-stack-medkits-and-pistols-in-backpack
I like this bug, it makes it convenient in storing items like Medkits and Stun Batons. It seems to work with any 2x3 item. I think it's balanced enough by the fact that they have weight and you can't carry too many of them without your weight becoming an issue.