Back when I was flying a lot for business (2006-2013) I bought a TSA approved laptop backpack so that I didn't have to take out my laptop. Every time I went to the conveyor belt, I unzipped it and laid it out flat.
At one airport, after I unzipped and folded out the backpack, the TSA agent took my backpack off the conveyor belt, zipped it up, and sent it through the xray machine. Of course, that set off the system and I had to wait for a supervisor to take my backpack and me for special screening.
As he was swabbing my laptop, he explained the rules to me. I nodded and said "uh huh" with each one, since I knew the rules. When he got to the end and said that their are certain backpacks that you can buy that you don't have to take the laptop out. I responded, yes, I know. This is one of those backpacks, and I had unzipped it and laid it out flat following the rules, but then the TSA agent at the conveyor belt (and I pointed to the one), took it off the belt and zipped it back up!
The supervisor got a horrified look on his face, apologized profusely, and had me go on my way. I then saw him walk over to the agent and explain the rules to her.
The whole point I was trying to make is that a prohibited item made it through security screening at about a dozen US airports until someone caught it. TSA doesn't catch everything.