So the scene: You're flying from Bogota, Colombia to New York, USA. You arrive, pick up your bag from the carousel, and figure you'll stuff your netbook you used on the flight into your bag now rather than lugging it through the airport under one arm.
However, when you open your bag, you see something that you certainly did not pack. Drugs/a bomb/live snake, take your pick.
Now you didn't pack these, but you're pretty sure the guys ahead who will x-ray / sniffer dog your bag won't take kindly to seeing them there. What are you supposed to do in this situation??
(Note, while this has not happened to me, it's certainly claimed to have happened to several people travelling from Bali, Thailand and other places - see the Schapelle Corby case for an example)