I brought a 3-inch credit card blade (acquired at a trade show, with some tech company logo on it) through the airport four consecutive times without even realizing it.
I found it while packing for another trip, and I decided to gut my overly-thick wallet and realized that I’d been carrying it around everywhere, including through courtrooms and other government buildings that X-ray everything I bring every time I pass through as a contractor.
So yeah, a knife inside my wallet went through about 50 X-ray machines at federal facilities completely undetected, and I unknowingly carried it through all sorts of places where it is extremely illegal to have a knife. They always scolded me though if I tried to bring anything made of glass, like a coke bottle or something, because it could be used as a weapon.