There are a lot of tricky (or dire) situations that you could get into due to the layout of characters on the keyboard (the QWERTY system at least).
For instance consider the two letters 's' and 'd'.
Any unix user will know that its almost sacrilege to type some commands without proper authorization, especially if the logging systems are setup perfectly. The admininstrator can take you to task - like cutting off ur email, not allowing you to login, deleting all ur files, or worst still hosting ur stuff for the whole world to see, ... - for such a misdemeanor :)
Imagine my consternation when I happened to type 'su -sh .' instead of 'du -sh .' ! :) My agile mind (which was actually not present and not working earlier, not that it ever does) ran faster than anything in the whole universe and I panicked to press the eternal savior 'Ctrl+C'.
I'm now waiting for the 'i-can-sell-ur-soul-now' call from the system admin :)