Playing as Keelyin with Slavery, I am able to arena battle my slaves. Originally this caused a 'huge' python error (at least a hundred, I would guess, windows popped up: I moved at least 25 and closed another 25 without any change in the stack), However, in reloading the save an attempting different variations of the arena battle, the error stopped occuring. Unfortunately I did not think to screenshot the error before reloading until it did so. In any case, if the slave is victorious it upgrades into a warrior with any promotions the worker may have had (notably the aptitudes).
Now that I think of it, this is a pretty thematic function: slaves fighting, and when they win they are freed and become a warrior. Is this intended or an unexpected function?