There is a well known problem of building a service requiring two different communities (e.g. sellers and buyers, teachers and students).

And sometimes the actual problem is that the distinction is made explicit, where it shouldn't be (once I had a problem, as I by mistake registered in one, and couldn't use the same e-mail to register in the second - even if there was some overlap).

Here it is an interesting example of the opposite. And also, a practical remark against political correctness :).