Yeah I encountered that with IAin being in the rear facing position. He was over 12kg but not ready to turn IMO. I actually rang the manufacture of the seat he was in & asked. They told me that the seat is tested way beyond the recommended weight. So I was more then happy to keep him RF even though he was over the 12kg limit for the seat in that position.

I would urge anyone to look further into it then just the label before putting their child in a adult seat/seatbelt set up.

Trust me, I know the thought of more money for a seat that they will outgrown before too long is hard but not as hard as what could happen.

eta - the seat I have my 7yr old in (Graco Air Booster) is up to 45kg
Spending the money doesn't worry me. I am acutely aware of the worst that can happen in car accidents. The only reason I haven't been using a seat is because I haven't found anything in Australia that goes over 26kgs.

Did you buy the Graco in Aus or from o/s?
From what I see the Air Booster Turbo goes up to 100 lbs but I haven't found it available on an Aus site. Only the standard model which only goes up to 26kgs.

Just because they test it at higher rates doesn't mean it's as safe. Other wise they would adjust the weight ranges in accordance KWIM?
As with everything it all amounts to the risks we are willing to take. Things would be much easier if a wider range of seats were available to cater outside the "norm".