I had one instance checking in at LAX. At the time, I had a roll-aboard/backback zip-together combo. I put the zipped-together pair on the scale and it came in at 18ish kilograms. I was told I was over the limit, so I unzipped the backpack from the rollaboard and put the rollaboard on the scale again. That came in at just under 7 kg and they tagged it as OK.

For those paying attention, that means the backpack was over 10kg. They didn't ask to weight it.