I accidentally flew with a knife in my backpack once, between Vancouver and Calgary.
A friend had dropped his knife in the car so I brought it back to school in my pack and forgot about it. My bag got searched at the airport, and my pencil case was in it, with a compass inside it. They thought that was the metal that appeared on the screen, and when I got to calgary I realized there was a knife at the bottom of my backpack...
Needless to say it isn't that hard to bypass security.