One simple sub that I got caught with a few times is an arm and neck choke. I was on top and the guy I was rolling with had me in full guard, and as we were fighting for better hand positioning, he slapped my arm across my body and shot up quick, wrapping a front arm and neck choke on me. Twice I fell for that before I started watching out for it.
Anyways, that kind of submission would be harder to catch on a player that has experience, but anyone can slip up and if you can get the arm across the body and get your choke wrapped up quick, it works.