One time on my way to Switzerland I threw a bag together really quickly and had forgotten to check what was at the bottom first. I had just moved and had thrown stuff in there. Mid flight I'm digging through the bad looking for something when I find my five inch Spyderco knife.
For about five seconds I was like fvvvvvvvvvvvck, I'm so screwed. I shut my bag and felt nervous for the next 7 hours.
Once I was off the flight I didn't understand how security didn't see it. It wasn't even hidden. Thankfully it wasn't found or I probably would be on the no fly list.