You can't empty the Trash or move a file to the Trash
Visit The XLab FAQs and read the FAQ on solving Trash issues. You can also try using Trash It! 5.1 to fix the problem. Or you can try this:
Open the Terminal in the Utilities folder and paste the following at the prompt:
sudo rm -Rf ~/.Trash
Press RETURN. Enter your admin password when prompted. It will not echo to the screen. Press RETURN again.
(Should anyone tell you this is dangerous or unnecessary, please ignore the comment.)
It is possible you had selected Empty then selected it a second time before the first command completed. This could happen if you had a large number of files in the Trash.