True dat, Gemmy. It's all about knowing where your fishes are, and IMHO getting to know each other. FWIW, I've been keeping venomous fishes for about 20 years now, and have never been stung, and I regularly work in tanks that have multiple venomous fishes in residence.
Lionfishes become very tame if you let them, which I prefer to having them "spooky" and darting around the tank when your hand is in there. The only lions I have "trouble" with would be our M-F fuzzy dwarf pair. They INSIST on circling my arm, hoping for a treat, but even so, I'm not really concerned. I've actually had one of our waspfishes perch in my palm a few times, and have gently lifted one of our Rhinopias out of the way when I was cleaning its tank (not that I recommend it, but I was curious as to just how tame they were).
Here's an excerpt from our lionfish care article regarding first aid for an envenomation:
First aid for a lionfish sting is immersion of the affected area in hot water (114°F) for 20 up to 90 minutes, or until the pain subsides, in order to inactivate the thermolabile components of the venom. The reason for applying heat to the wound is because lionfish venom is composed of heat labile proteins, and the heat actually denatures the venom. Please, don't use scalding-hot water, as the resulting burn will likely do more damage than the venom. To ensure the proper temperature have a cooking thermometer on hand.
Don't worry, unless you happen to be allergic to the venom, or you get stung by a stonefish, your life really isn't in danger from the effects of the venom. That being said, you need to guard against secondary infection of the wound as well as make certain that there are no pieces of the spine left in the wound, which can cause infection. A tetanus booster is recommended if it is past due. To that end, you may want to seek professional medical assistance, just in case.
HTH