Or think about April 1st, when they set a Access-Control-Location: * header on google.com because someone wrote the com.google easteregg.
Read the post from the SoundCloud dude from yesterday to find out how to do software management properly (hint: modularization is everything)
The issue was that they wanted to load the page – with the user logged in, etc – on com.google. For this they implemented an explicit URL parameter that would allow this.
The horror!
Neither HN search nor Google search show anything to "modularization is everything".
http://philcalcado.com/2015/09/08/how_we_ended_up_with_micro...?
(That question mark is part of the URL)
It explains how even in a small company modularization can help extremely. Now look at Google, where some issues (like the google.com april 1st XSS issue) were only fixed after outsiders mentioned it.
Usually internally the team responsible for that part should have cought that.