1. Have a weapon and few full magazines in inventory.
2. Reload a weapon (I used USG), holding the R button.
3. Try taking each magazine from inventory into hands and unload it from ammo (or just keep moving them around in inventory)
4. If you find a bugged magazine, take it into hands, press G then LMB and throw it.

It seems like the function which chooses the magazine to be reloaded, chooses different magazines on client and server, which causes them to be desynchronized.