I wasn't paying attention when I typed:
mstsc /v:name.domain.net /console
Unfortunately, I was already sitting in front of the very machine I was trying to reach with terminal services. I took over the console, all right - the screen blacked out. I couldn't get Ctrl-Alt-Delete to provoke a response. If it wasn't for the lights on the case I'd have thought the computer died.
It takes 30 seconds or so for the un-credentialed new connection to timeout and return control to the real console.
It's a very long, worrying 30 seconds, though. I just wanted to share my anxiety.