Say my inventory is full, and I go to reload my 30 round mag for a 15 round mag. Obviously the inventory gets on new free slot as the 15 round is remove but then there is no more room for the 30 round to go back in so it is lost. I checked the floor and other places. This has happened before to me with the M4A1 as a changed a 30 round for a 10 round and I lost the 30 round mag.
This bug is incredibly annoying as it can happen easily when you have your gun out and you drink from the a water source and accidentally click the Reload option instead of Drink.