One of the cruelest things you can do is a filename that consists only of a combining diacritic (without a glyph that it could combine with). Will break outputs of various programs (starting with ls) in sometimes hilarious ways.
If you're trying it out now and cannot figure out how to delete it: "ls -li" to find the file's inode number, then `find -inum $INODE_NUMBER -delete`.
Wow, that's really horrible. I have a file sitting around with a couple of newlines in the name just so I can see how many programs don't cope with it, but I hadn't thought of using a lone combining diacritic.
If anyone wants a command to make one, try
touch $'\U035F'
(using U+035F COMBINING DOUBLE MACRON BELOW for no particular reason, see [1] for more)
If you're trying it out now and cannot figure out how to delete it: "ls -li" to find the file's inode number, then `find -inum $INODE_NUMBER -delete`.