Following a training dive during the safety stop I had Sylvester do the valve shut down drill, we always do this at the end of every dive. As he was about to do this I noticed that is left post was leaking small tiny bubbles. So I had him just shut that post down. During the time I was having him shut his post down he was trying to tell me that my left post had a leak. Minor short bit of confusion created here but quickly solved none the less. His was at his first stage and mine was the burst disc plug. As it turned out he shut down his left post reg and I shut down my isolator and breathed my short hose. We surfaced and used this as a training point and called it a day. His problem was the O ring on his reg and mine was a loose burst disc. He changed the O-ring and solved the problem I tightened the burst disc and all was well. Who would figure this and what are the odds. So next time you are thinking as to why we do cetain skills certain ways THINGS LIKE THIS SHOW US WHY- YOU THINK. Sylvester is working on his cavern cert. From the pre-dive S drill all was well and good to go.