From GreyMario on MCF:

I've discovered a game-breaking bug where moderate latency issues (cross-United States distance) can cause the Lunch Bag item to feed you without consuming an item within.

Simply hold down the right mouse button while the bag is closed. This opens the bag (clientside) and asks the server to send you the container object describing its contents. While you're waiting, if there's enough lag, your client then decides "I'm holding and using an open lunch bag, feed me". The server gets this while it's busy sending you the container object, and says "okay, here. I'll close your bag item and show you what's inside. okay, I'll feed you this item from inside this bag.". HERE'S where it gets interesting: you already don't have the open bag. Apparently the game treats the open bag differently from a closed bag (i.e. as a separate item) and when the server is asked to feed you an item from the open bag in this situation, it attempts to modify the now nonexistent open bag item after feeding you.

The final result: Food Journal reports that you ate the most efficient item from the Lunch Bag. The Lunch Bag still contains that item. You receive the full benefit of the food you just consumed.