I gave a #16 key to a male dwarf slave earlier, and the slave wasn't set free. I mentioned it to a party and one of them said they have had problems with that before. I know i gave him the correct key, as i used the wiki before i gave it to him. This seems to happen only every now and then, but it's still not acceptable that i turn over a quest item and lose it for nothing.
http://wiki.project1999.com/Slave_Keys