In my case, it was a 16 year-old bottle of scotch that I carried on to my flight from London after purchasing it in a duty-free shop at Heathrow on the "safe" side of the security wall. We connected through Vancouver on our way to SEA, and de-boarded the plane through a hermetically sealed hallway that led directly to Vancouver airport's U.S. security checkpoint (which is still inside the secure zone of the airport). So it was literally impossible to arrive at the next checkpoint with anything that I was carrying from my London flight to not be in compliance for being a carry-on.
Yet I was told the bottle didn't comply with the 3.4 oz rule. Duh! But this bottle has been in the secure zone the entire time I possessed it, and in fact, it still IS in the secure zone, I tell them, so why the double jeopardy? Sorry, rules is rules, they tell me. They did offer me the opportunity to check my carry-on on-the-spot, but since it was a tight connection, I didn't have time to do that and actually make my connecting flight. So I left a perfectly good bottle of scotch on the counter, and hurried to make my next flight. I'm sure the folks at the checkpoint disposed of it properly, since it was such a grave threat!
But they never were able to explain how an item which was apparently harmless enough to be carried onto a 10 hour trans-Atlantic flight, was transformed into such a huge security threat simply by being taken off that airplane (while still in the secure zone), and then could somehow magically be transformed back into an inert object simply by being checked and placed in the cargo hold of the next plane.