... Has any one ever got a squeeze in a modern hat? just wondering if is possible because of the demand reg not free flow. (I know all the modern hats have one way valves) ...
I partly addressed your question but realized I got off in the weeds with the answer. Regarding the demand regulator question, the only way I can see a squeeze is if your check valve fails AND the regulator seat also fails/leaks (assuming the freeflow/control valve is closed). Debris in the hose can cause both to fail, but you will notice the demand regulator leaking where not many people test their check valves. An upstream demand regulator is basically an in-line check valve forced open by a lever and diaphragm. The risk is further diminished in a BandMask or if your neck seal is pointing up in a hat as already described.
The greatest concern of squeeze in my experience is on closed-circuit, surface-based, mixed gas systems; sometimes called a push-pull. In that case there is a negative biased back pressure regulator that could fail open, usually in the bell. Since any crap that goes out your exhaust (exhaust regulator in this case) could also go through the back pressure regulator, the chance of failure is pretty real. You definitely want to be careful about the direction your neck dam points on one of these. It is also important to keep the water separator filter upstream of the back pressure regulator clean.
In the incredibly improbably event that you did get a hat or mask squeeze, it would be much faster since there is so little gas volume. I am just guessing here, but I would think a catastrophic squeeze in a hat with a neck-dam or rigid seal mask would involve lung tissue heading up the hose PDQ.
So, blow out your hoses before use and keep them capped when not. If nothing else, it can save some maintenance.