That was a lovely 'gotcha' for one of our instructors a while back. He called up for a radio check while I was doing my turn volunteering on the radio. All I could hear was what sounded like his voice in the background, very quiet. Quick as a flash, I said 'have you got your headsets plugged in the right way round?' 30 seconds later he came on, readability 5, 'how's that now?'
So how did I know? Been there, done that!
TOO