Those moments when you hit enter and then slap your forehead. Some notable examples: recursive removal of a directory under which various other boxes' filesystems were NFS mounted; attempts to transfer filesystems using tar with an absolute pathname, like this:

tar cf - /path/to/directory | ( cd /new/path && tar xpf -)

Feel free to add your own horror stories.

scripts to add and remove users often remove home directories. Sometimes with rm -rf ~username. Which is bad enough, but think about what happens when that user is root, and its home directory is /.

When you make a mistake in the pam.conf file, it's not a good idea to log out before you test whether or not you can log back in.

...

Never ever ever give the wildcard expression .* . This will cause more hurt than you could possibly imagine. Why? Because .* always matches .. . If you must do something unsavory in this vein, try .?* instead.