Forgetting to put the volume loudspeaker up when removing the headset is like forgetting to put your socks before your shoes. I’m not saying it can’t happen but it’s unlikely. It’s like an automatism for me. When we agree to remove the headsets, it’s loudspeaker on first and only then headset off. In the airspace I fly, there is always someone talking every 10 seconds. So in case there is an usual silence, I would know pretty fast something is wrong. xD I don’t recall being on that situation. And yes I do forget and do mistakes like any other pilots. But this one is easily avoidable with discipline.