The one's left don't know all the code (how could they?), but were forced to change many things about the site at a "just do it" basis. This error didn't happen because someone was too stupid to remove the code, it did happen because the connection to another thing was removed and the failsafe on the landing page doesn't have exponential backdown built in, not something you can necessarily know or investigate before, when an executive breaths down your neck and wants you to just do it.
This is about the new managment, not about engineers.