For a while I’ve been refilling standard camping iso-butane/propane cylinders with butane from various types of containers. Note: do NOT refill with propane unless you have a chemical engineering lab at home, as the propane vapor pressure is high and overfilling can lead to a cylinder rupture (and possibly your very own Hollywood special effects explosion, but with you at the center).
I use the following to convert from portable stove butane cylinders to a Lindahl valve (under $3 on Ebay or AliExpress):

I then use the following to connect Lindahl-to-Lindahl (under $3 on Ebay or AliExpress):

This works fine if the I empty the source cylinder into the receiving cylinder, just unscrew the adapter off the now-filled cylinder when done.
But what if you don’t completely empty the source cylinder? I figured it would be easy to just unscrew both sides very quickly and have minimal leaks.
Wrong!!!
I tried unscrewing both simultaneously. I got one off and then my hands got sprayed with extremely cold and rapidly evaporating liquid butane. I continued unscrewing as fast as I could, but by the time I was done one of my hands got sprayed a lot and it stung a fair amount. Fortunately, I was fine in about an hour, with no evidence of frostbite. Also, I was clever enough to attempt to separate the two cylinders outside the house.
A solution is to use an adapter with a valve, like this one ($4.34 on Ebay incl. shipping):

This seems to work fine, nothing comes out the other end if the valve is closed. You can spend up to $35 for similar adapters, I don’t know why they are any better.
Side note: my strategy to move butane from one cylinder to the other quickly is to first put the receiving cylinder in the freezer for a while, then VERY CAREFULLY use a heat gun on the source cylinder. I gently heat it on the lowest setting for 5 seconds or less, then repeat once in a while, always touching the source cylinder to make sure it doesn’t get too hot.


