This was brought to mind by this question:
What are the rules about joining your co-workers (or the like) for a meal in a non-kosher restaurant, but bringing your own food? Is it prohibited as mar'is ayin (because people will think that you're eating non-kosher food along with them, or that the restaurant is in fact kosher), or is there not a problem (because your bag of food, or whatever, makes it plain that you're not eating from what the restaurant is providing)?