Changing a tube on the side of the road with deep rims, I pulled out my spare tube with simple tubular extender already mounted and found the hand pump wouldn't generate enough pressure to push down the valve (which was definitely left open). Had to take the tubular extender off and push down the valve manually to free it. Has happened at least twice.
Every once in a great while when topping off a tire, I will notice that a valve needs extra force to push it open after unscrewing the lock nut; I have no idea what causes that, but it's never an issue if the valve is out at the end of the extender.
I always take a bent paperclip with me for that reason: also necessary when you want to let out rest air out of the tube, that is when the tube looses air but you can't get the outer tire off because there is still rest air in the tube.
I furthermore secure the lock nut in an open position with locktite.