Sometimes when I am emptying the trash, I want to put another file into the trash, but OS X tells me I can't because "You cannot move any item to the Trash because it is being emptied." It happens more often than you might think, especially since I turn on "Empty Trash Securely," and often delete video files hundreds or thousands of megs in size.
However, if I move the want-to-be-deleted file into a Stack in the dock, I can then drag it from the Stack into the trash immediately, even while the trash is still emptying.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090109105942758