I didn't see this in the Bug List, apologies if it's a dupe. If you have a building with specialists assigned to work it (library, for instance), and sell the building, you apparently sell the citizens into slavery along with it. You should be able to verify this anywhere, but see the attached save. Cherokee has 15 population, but only 13 available citizens if you count all the workers and specialists. You should also be able to recreate the condition in any one of the, ah, numerous cities provided. Saving and reloading does not clear the condition. I've not played further along in the game enough to know if it is corrected by a change in city population. Reassigning everyone, telling the governor to reset tiles, etc., does also not clear the condition. Be forewarned, I'm ICSing on a Huge map, so it's probably crash-prone. I'd recommend switching to the strat view immediately on load.