There was an excellent thread on PerlMonks a few months back about a sysadmin messing with their users by making fake shells (check SuperSearch). My suggestion was to write something that made 1/10th of the presses of the 'W' key be eaten, so it didn't work all of the time. This would be really cool as it would be broken on *multiple* keyboards, and possibly you could do this to affect only certain IP's. For more fun, make random hits of the 'W' key print multiple 'W''s. This might be harder to implement than it looks, as you have to do a very good job of feeding the 'real' shell.

Be careful about your job, though!

Another one, oft thrown about is to change your user's favorite shell in /etc/passwd to /usr/bin/wumpus or maybe one of the famous 'Text Adventure Shells'.