Sometimes, when a player is observing and the server changes the map (time limit reached or by the newmap command), the observers' names will not be shown in the scoreboard after the map change. The players can talk, but their name will not be shown in the console if they do so. Entering "players" in the console will show the correct names though. It can be fixed by joining a team. As soon as that happens, everything works as usual. As far as I know, this started happening in server version b41. It happens to clients with b40 and b41, so that doesn't seem to matter.
I'll attach an example screen shot.