So right now you are able to overfill ammo boxes, causing massive desync for as long as you have it in your inventory.
I'm assuming it works by counting the number of stacks of ammo in the box, rather than the rounds, but by unloading a coupled mag, you can actually get stacks of 60 rounds.
EDIT: Changed title. Also, it doesn't seem to affect every player. A member of our squad had stacks of 60 in his ammo box, and it only caused some of us to desync. The rest were fine.
EDIT 2: Seems to happen when players have too many yellow protector cases on them too. {F29744}