Every time someone recognize a masked Diabolik without checking the face for masks.
Even more awesomeness for Elizabeth Gay. Everyone else recognized Diabolik because he said or did something the character he impersonated was not supposed to say or do. She recognized him by looking around where she expected him to prepare a heist and looking him well.
Also, the man who created a machine that could detect Diabolik's masks. Sadly, he didn't survive his first story...
Diabolik's masks, capable of perfectly reproducing human skin. Why? Well, many tried to copy them, but not even the man who taught him chemistry succeeded in making masks that could both reproduce human skin and survive more than a few hours.