In cold water a "freeze flow" an occur. Turning the valve off for a few minutes is often sufficient to allow the reg to re-warm so that if turned back on it may be useable again, particularly if you have ascended into warmer water.
In that case, switching to an alternate source (buddy or pony) and turning off the valve to the freeflowing reg will preserve the gas and can make it available for a safety stop and/or in general increase the gas reserve for the buddy team.
I had this exact thing happen to me while in my Adv. Nitrox course. I did an emergency shutdown drill of my primary regulator when my secondary started free flowing violentlly. I was in the quarry at 95 ft in 42 degree water. I breathed off the free flowing regulator, sorting the situation out for a few seconds, until my instructor decided to give me his regualtor whilst I turned off my secondary & turned my primary back on. The good thing was I had just covered the skill in my ITC class the week before. No panic or anything. I just dealt with the situation as it was. Very valuable experience for my first ever true freeze up & free flow.
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