Remember the crazy smart contract challenge from the Real World CTF? Here is a video specifically talking about the backdoor method used. Together with @ret2got we finally understood it.
Acoraida Monica - Real World CTF
Part 1: https://www.youtube.com/watch?v=ozqOlUVKL1s
Part 2: https://www.youtube.com/watch?v=RfL3FcnVbJg