like what was said above, the first stages pretty much have to fail open. There isn't a whole lot that can cause the first stage to not deliver air, it goes against the design. Since it is a spring going against the pressure of the tank, that spring can't be strong enough to block air flow, though if it is weak enough or backs out it could be rather difficult to breathe, but not impossible. Either way the likelyhood of the main spring failing in a first stage during is 0. The only time that could happen is due to operator error, i.e. not getting it serviced basically ever and the spring corrodes, odds of that are also pretty much 0.
Second stages can fail closed, the upstream design of the Jetstream and Xstream can and will fail closed in the advent of a HP seat failure in the first stage, IF and ONLY IF, you are an idiot and don't use a Poseidon hose on a first stage that doesn't have an OPV. Which they tell you about in the manual, and again is your fault. These second stages will never fail to freeflow due to IP creep which is the most common regulator failure and will in fact lock up to the point that you can't breathe off of them. The OPV in this case releases the IP down to where the second stages are functional as normal and as such are a better design than a normal second stage where it will freeflow into your mouth. Bad news there. Again, operator error is the only thing that will cause that second stage to lock up, and if you think you're holier than though and choose to ignore the manufacturers recommendations like that, well you deserve to eat it.
The case that Steve pointed out was again operator error, the reg didn't fail, the idiot who serviced it did it wrong, so again not a case of the reg failing. The "normal" second stages are designed to fail open, the main cause of failure is IP creep in the first stage, so they will bleed the pressure off to prevent hose rupture, this will usually come from your primary as it is generally tuned to be more sensitive than the secondary. I have seen many regulator failures, but have never seen one fail closed, not to say that it can't happen, but under normal operation with proper service the only thing that should go wrong is an O-ring failure, or the HP seat fails, both of which will still allow you to breathe, albeit with a great deal of difficulty.
