Ever mis-typed sl instead of ls on a Debian GNU/Linux box. Did that and was baffled to see a nice ASCII art on the screen.
Check it out if you have a Debian box around.
$ apt-cache show sl
Package: sl
Priority: optional
Section: games
Installed-Size: 70
Maintainer: Kenshi Muto
Architecture: i386
Version: 3.03-13
Depends: libc6 (>= 2.3.1-1), libncurses5 (>= 5.3.20021109-1)
Filename: pool/main/s/sl/sl_3.03-13_i386.deb
Size: 13804
MD5sum: 7cce44dbc0824d4ab97bc86f088524da
Description: Correct you if you type `sl' by mistake
.
Sl is a program that can display animations aimed to correct you
if you type 'sl' by mistake.