There is one tricky issue I'm dealing with now.
In the system working with data there are filters. One of them is shown in the form of list with "Select all"/ "Unselect all" buttons and user can filter out some settings he needs (as the example I would explain it on colours): he can select options like red, blue, green etc. and one special option meaning "no colour" - does not equal to "transparent", but more to "we don't have data about this object's colour" or "this object should not have a colour".
So the question is: how this option can be marked in the list together with other options, but not "lost" among them? Basically, how can it be named? "No data" is not clear, "no colour" does not describe properly, that we simply don't know, if there should be one...
Putting this option apart or creating another "level" of filtering defining if the object should have a colour or not is not a solution. User should be able to select at the same time e.g. "no colour" and "red".