Siemens Step7 code block protection (PLC's).. It was designed to lock code that you don't want others to be able to read. All blocks are in a dbf file, so you just need to find the block record and uncomment one line, voila - source code available.

Given the massive use of Siemens PLC's on plants all over the world, and the simplicity of hacking via S7 protocol, usually Internet connected, it's a breeze to steal or modify the controllers code with possible critical implications.

Enter Stuxnet.