Sometimes drag & drop fails to drop and gets stuck there. 
The drag icon continues to be drawn, indicating that it hasn't finished the D&D operation. All mouse button inputs are ignored (mouse positioning still works though). Very limited keyboard input works: e.g. switching workspaces works, but not alt+tab, alt+f4 et.c. Ctrl+alt+del would bring up the shutdown computer dialog but it automatically closed, hidden or something because it is only visible a very short time. The only way to get out of this has been to make a hard shutdown by cutting power. I've found no way of getting a terminal or switch application or anything when this has happened.

This has happened several times for me on Ubuntu 10.04 when I've repeatedly dragged video files from the File Browser to different media players (this last time it was Totem Movie Player 2.30.2). I've found no reliable way of repeating this behavior.