Short and simple: Use a checklist to prepare your rebreather. Complete a five minute pre-breathe with a blocked nose while safely seated and paying attention. Never dive a rebreather with a known fault.
Most rebreather manufacturers provide students with a last-minute checklist that can be completed right before jumping off a boat or even in the water prior to descent. If your rebreather does not have such a list consider reciting a short mantra as you approach the “danger zone” before a dive. The danger zone could be the top of a set of stairs leading to the water. It might be the transom of a boat before doing your giant stride. The danger zone is the place where unconsciousness could further result in drowning. When I owned a Cis-Lunar Mk5P, I made little…
Kudos to Curt Bowen from Advanced Diver Magazine for taking time to document the worst case scenario of catastrophic gas loss underwater. His article tests different complete gas failures cause by low and high pressure hose ruptures and burst disk failure. He and his team quantified the amount of time it takes to completely dump a scuba tank. The “Life Ending Seconds” video and article remind us how important it is to plan conservatively and rehearse valve shutdown drills.