During map-restart, the client attempts to send the map_restart command to cgame. Before doing so, it calls Con_ClearNotify(), which has an instance of Cmd_TokenizeString() that messes everything up and results in sending a 0-length empty command to cgame instead. The result is fairly minimal and restarts appear to work despite the issue, but in Tremulous, where we noticed this, our cgame prints a warning message that caused us to track down the bug. 

If you look a few lines up at the code for "cs", the same issue occurs with handling the "cs" command and the same fix is applied, so this should be pretty open-and-shut. 

Thank Amanieu.