That box cutter story reminds me of one:
Friends and I were bush walking around Canberra, and decided to go to Parliament House one day, to save on bags we just took the one hiking bag. Problem was that persons bag had a pocket knife that we completely forgot about so it went through the bag screener, the person got called aside and then realised what was in the bag, luckily nothing came of it, just had to have the bag cloaked.
My dumbest one was upon arrival in LAX after a SYD-LAX flight late 2008. I had deodourant in my ziplock bag. As I entered the terminal after immigration (UA) I put my bag through the screener and get to the end and my bag isn't there, I get called aside by this woman who asks if that's my bag, she then pulls out the ziplock bag and asks what it is, I just tell her what it is and that I cleared security in Sydney 16hours ago and completely forgot it was there. She kept me waiting a while but she let me through.