You may be minding your own business and discover that your clipboard has stopped working. You try to copy something to the clipboard, and it's not there. You try to paste something from the clipboard, and nothing comes out. What's going on?
The clipboard is a shared resource. (More specifically, shared among programs that run on the same desktop.) The window manager automatically closes the clipboard if a process terminates while it has ownership of the clipboard, but if a program opens the clipboard and simply forgets to close it for whatever reason, the clipboard will remain unavailable to other programs until that program exits.
If you find yourself in this situation, and you want your clipboard back, you'll have to start exiting programs until you find the one that has been holding the clipboard locked. The Terminal Services Team discussed one case where the
rdpclip program can become the "bad guy" that locks up the clipboard. (There's a second part to the story for those who can't get enough.) Or it might be a Virtual PC Virtual Machine Addition.