Sure there is. If they ask you to do something that they would legitimately ask an employee to do, there's a good chance they're looking to get some free work (or maybe the interview is genuine but they want to get free work out of the people they decline). As an employer, if you're trying to come up with a good work sample to request, you need to request something that's obviously not "real" work. In the case of the marketing manager, you could ask for 2-3 ad campaigns, but have them be campaigns for an obviously fake brand (ideally one in a different line of work entirely).