i’ve, unwittingly, taken a multi-tool which has a knife on a few flights, didn’t even realise until i got back home.
but i’m always wary of taking parts on planes. i opted to post a stem to sydney in lieu of taking it on board.
Good clubbing potential I imagine… 
Yeah, I bought a stem and seat overseas and was reluctant to take them as carry on on the way home. Last thing I wanted was Heathrow security taking my shiny new Nitto stem.
I bought a skateboard deck (just the deck, no wheels, trucks or anything) in Japan last year. Took it as carry on from Japan to Brisbane, but I couldn’t take it as carry on from Brisbane to Adelaide. Crazy.
But I’m pretty sure Singapore Airlines, or maybe Virgin Atlantic let you take a bike onboard at no additional cost because it’s covered as “sporting goods”. Like skis.