Ds1 has always fancied himself as a bit of a houdini. To date this has involved slipping his arms out of his car seat straps - dangerous and annoying, but he does put them back if yelled at.
Today he excelled himself. Today I was driving along at 60 mmph when ds1 appeared in the front seat with a cheery 'hello mummy'. He had undone the buckle on his car seat and clambered through to the front. I knew I'd done up the buckle properly, but began to doubt myself. I put him back in the seat, checked the straps were as tight as tight can be, the buckle was fine etc, then 2 minutes down the road he'd done the same again.
It is a Britax Duoplus Isofix seat, if that makes an difference.