This happened again and this time I actually took a screenshot, since I guess it says more than a 100 words - and because I assume we may have been discussing different things here.
[img]http://megaglest.pzt.me/dx/subdomain/megaglest/img/5/2/0/5202.jpg[/img]
This situation seems to occur after "Reload last game" was used (I'm not sure whether it only occurs then), and when the server admin switches things around on the server, such as replacing a network slot by a CPU player, while a new client is in the process of joining.
The text on the image is not exactly right: This forth slot used to be open earlier (due to Reload last game loading an 8 player map), but it was probably closed before the new player started joining.
This is not an off-by-one, we're had situations where a 4 player map was set, and there were free network slots (0 and 1 were taken, 2 and 3 were free, all others closed due to a 4 player map) and a new player joined on slot 6 and could not move away from there. Also reconnecting does often not help in the first attempt - you may end up in the same closed slot again, and the server admin may even miss you are there because only this one button is displayed - not even a player name.