If you leave the picnic tap hooked up at 40PSI, you can indeed wind up squirting your liquid out.
I re filled my CO2 last weekend and since the beer was a little flat from the pressure going down when the last tank went empty, I hooked up the new tank and cranked it up a bit.
I intended to come back in 20 minutes or so and shut the gas off, but it was late and I forgot.
The next day I opened the freezer and saw that I had the picnic tap still on the keg ( it was late and I didn't look ) the freezer was sitting with about 4 1/2 gallons or more of my 5 gallon keg pooled in the bottom of the freezer.
That was an expensive batch of beer and a full bottle of CO2 that was gone.
I would turn the pressure down after the water is carbonated and shut the gas off when not in use.