The heartbeat code was originally written by a German volunteer. The code was written as an improvement to openssl and submitted to one of the core guys, one of a small handful of volunteers, who look after the openssl releases. Well, these guys also reviewed the code and what we know as an error now, was overlooked. The new code was subsequently added to the release and the rest is today's news. Just goes to show, even the most diligent programmers can miss something unfortunately this little error caused a huge hole in the software. Even more, this 'bug' has been around for a couple of years already when the version of openssl was released with this heartbeat code.