If you have a weapon in hand (in my case first an M4 and then an AUG...)
and drag a canteen to the hand slot and replacing the weapon. The weapon should just be auto put down on the ground. But instead it disappears and is nowhere to be found..

This has happened to me twice, lost my M4 and AUG because of this bug. And no it's not because of small spaces and the weapon appears outside or something like that. Both times was on big open grounds.