In training you learn to take off and don your rig underwater, then you forget about it. Then one time I got in the water and found every time I put air in my wings it came back out again somewhere I couldn't see with my mirror. I put my penny in the swear box and switched to secondary. On surfacing, it was pointed out that if I'd taken my rig off and looked, the butt toggle pull to the wing was caught up in something
. Of course, if my buddy hadn't been such a merchant banker he would have pointed it out when he saw me looking. The exercise is there in the course for when your buddy's not watching your butt, though.