According to this BBC News article, “An unwitting passenger arriving at Japan’s Narita airport has received 142g of cannabis after a customs test went awry, officials say.”. Against regulations, an agent hid the training sample in a real passenger’s bag rather than a training bag. Apparently he has been doing this regularly.
Imagine the scenario at some other checkpoint: “But Officer, I have no idea how that got in my luggage! Oh, wait … I bet Tokyo Customs put it there! Yeah, that’s it!” Shortly after that you hear an officer’s cynical laugh and the click of handcuffs on your wrists.