1. placement of the SPG on the right hip D-ring. Takes too many steps for a diver to check gas and isn't visable to buddies (so they know your gas supply)
2 buckle placement , the buckles are placed on every diver the same so the divers buddy can remove the rig in an emergency?. I would never even try to go for a buckle in an emergency. I'd go for my shears to cut the gear off. this crosses every agency and gear configuration. don't look for any buckle! just get to cutting
3. rock bottom calcs, way to much math and there are easier ways to make sure the team has enough gas.
4.false sense of security. just because you've trained with the same agency doesn't mean you can count on another diver being there when you need them .
5. the pony bottle theory. If you don't need it don't take it ????? how does anyone know the outcome of any dive? I think if you might need it take it , it's your life.
6. no solo diving. well everyone should be able to solo dive . self reliance is a must underwater.
there is more, but that should do for now. no need to get into gasses , decos or any other issues that come to mind