Having worked in Xbox Live I often get questions about its past.
Since I recently moved away from Microsoft and Washington to join Ericsson in California I recently did a number of interviews. And it was once again proof that most companies do not know how to interview in my opinion.
In a lot of older web applications as well as newer RESTful services I've seen it fairly common to have identifiers such as user and order IDs just being an auto incremented primary key from the database. This approach has the problem of revealing your total number of users/orders in a certain time period. Information your competitors could use against you!