When i instaled new beta patch for ns and tryed to join a srv i had a really strange thing, couldn't move, turn mouse but i heard myself moving in rr. Here's the screenshot i took:
http://img.photobucket.com/albums/v233/dragon143/huh.jpg

Similar things happen when you alt tab while connecting to a server.

As princess noted, this is a fairly well known issue, that has been happening for quite a while. Basically if NS is minimzed (either intentionally or because of something Steam/Windows is doing) while you are first joining the server this can occur.

The only work around is to quit out and restart NS, and the only thing you can really do about it is to not intentionally alt-tab while you are waiting to join the server.