You may think you know what's on board, but there could be surprises lurking in your fellow passengers' carryons and stowed luggage ... like scorpions, human heads, and squirrels. Whoa, and some nice acid dripping from someone's leaking suitcase onto your face?! Now I'm feeling pretty lucky not to have been sitting near these folks.
The worst I ever carried on board was a couple of geode bookends I was bringing my Dad for a present. This was way pre-9/11, but the X-ray guy got pretty excited about them -- guess they looked more like bombs than bookends to him. But I did put them under the seat so as not to brain someone if they fell out of the overhead. If there was anything super-strange on any of my flights, I remained blissfully unaware of it. I really hope to keep it that way, too.