[goose]

Regarding the mixing up of gas-in and liquid-out on ball-lock kegs, the posts are different sizes so a gas quick-connect won't fit properly on the liquid side (and vice-versa), although it can be forced. You just have to remember that if it feels wrong it probably is. A little food grade silicon spray will make the O-rings last longer and make connecting/disconnecting easy.

From my experience, you can get the gas-in connector to go on to the beer out side fairly easily and it comes off easily.  I used to crank-and-shake carbonate beers this way years ago before I came to the light and rejected the dark side of the force.
The other way around is more difficult.  T he beer out connector will not easily go on to the gas-in post and if you force it on, you will have a b**** of a time trying to get it off (I had this happen once and nearly destroyed the connector trying to pry it off.  Hence, the use of different colored o-rings as I mentioned previously.

[JT]

I'm a ball lock guy, and only recently discovered the different colored o-ring trick to avoid gas/liquid confusion. 

As for which type is better? The one that is full of delicious beer. 

[Robert]

I don't know if this is part of the confusion mentioned here.  But.  I've always had Corny ball locks.  I've never confused the posts, because of course gas is notched or has a filleted base, liquid is smooth,  and it is actually pretty difficult (though Goose notes the near exception) to cross up the QDs.   But my relatively new feremnter (otherwise love it) is a newly US-manufactured, 10 gal ball lock which, if I understand correctly, may follow the Firestone model -- and UNLIKE  the genuine Cornies and new copies, the posts have different threads, and cannot be put on the "wrong" side.  So if you've marked your kegs with a spot of paint or whatnot,  you can't possibly get them reversed after disassembling for cleaning.  I personally find this monumentally annoying, but that's me.