Handing back a bomb
G-CPTN:
I think the security screeners actually did a reasonably good job there. They found the device, then used some judgement to see if the guy was a terrorist or if he indeed just forgot about the device. I think it's possible to judge from the reaction of the guy if he expected the device to be in his bag.
I once got told a story of a guy who flew with a set of shuriken - he had got them as a present, stowed them in his camera bag (seems to be a good candidate for such stuff, as most people don't pack and unpack it frequently and forget about stuff in there) and forgot about them. He didn't get caught, but I'm told it was definitely an "oh sh*t" moment when he unpacked his bag for some other reason and found them...
I, myself, once brought a waiter's tool (one of those things with a bottle opener, a corkscrew and a knife) on a vacation trip and back - I had forgotten to unpack it from my carry-on after a non-flying trip somewhat earlier and only found it after getting back from the vacation... oops.
I'd be more worried about some attacker detonating one of those pipe bombs in the queue before security... On the other hand, wasn't the bomb that brought down Metrojet 9268 roughly that size?
What I'm trying to say, I believe bringing banned stuff on planes happens more frequently than most people think...
(Disclaimer: I'm just SLF)