Ever since I started using Linux the dyslexic in me have tried to run sl when what I really wanted was to run ls for listing the content of a directory, it is a typo that we all have made at some point but what would happen if sl was a command?.
Fortunately for our entertainment there is an application with this name and it consist of an ASCII train that pops of on your terminal, it is perfect for practical jokes, Just imagine, you install it on a busy server late at night and wait to hear users complaining mid morning.
Here is a quick video that shows you how to install it and what does it look like when executed:
In case that you can't see the video you can try it your self by installing it like this:
su - yum install -y sl
As you can see it is harmless or at least it brings less mayhem to the table than an alias to rm -Rf /*, In fact I found about this through a co-worker who brought it up when we were talking about practical jokes.
You can install in on OSX through brew too.