The adventures are contracted to deliver a shipment. The pay is great but the route is dangerous and the time frame he needs the package delivered in is tight. The package is sizable and so is the fee for care and delivery of the product unopened, undamaged and in a timely fashion. The players are given 5% of the payment up front. When The players make the delivery, much to the pleasure of the customer, he hands the party their payment and instructs his people to open the shipment. Once opened he flys into a fury demanding his payment back, the container is empty.
The joy of this scenario from a GM angle is finding the rabbit. The players are out the cash, and their rep is now tarnished. Did the shipper or the customer screw them? Did someone switch the cargo while they were on the road? Were they set up for failure? The list of possible reasons goes on.

On a side note this Side Trek could be run just as easily in numerous genre.