We've just encountered a serious bug. If you fill a bag with items, and then trade it to someone, they will NOT recieve the items inside the bag. Now, you can trade the bag back, and everything will be back in there.

We just lost a lot of items because of this. So from now on..

DON'T PUT LOOT IN A BAG AND THEN TRADE IT TO YOUR MASTER LOOT

Amendment: It seems that there might be a way around this: apparently, if you put an empty bag inside the bag with all the loot, then it'll work fine.