There have been numerous times at work that I've attempted to use Remote Desktop to get into a server, only to find that some people have left open their sessions, impeding others from doing work. If this is the case you would recieve the following message:
Terminal server has exceeded maximum number of allowed connections.
What's worse is that there really is no way to find out who those people are to tell them to log off. That is, if Microsoft hadn't incorporated a back door to solve the issue. In order to logon, use the following command (subbing 0.0.0.0 for the IP address of the server:
mstsc -v:0.0.0.0 /f -console
Just enter that and you should be able to login, disconnect the session hogs, and go about your business.