Here's the message I would leave with y'all:
"No sparks, flames, or smoking. Shield eyes- explosive gases can cause blindness or injury". Just like it says on the label. Grinding is a no-no (and most of us have probably done it).
Don't over-think it. It's not about a missing vent plug, electrical/ thermal conditions, density of gas inside the battery. Just follow the rules, and nobody gets hurt. K.I.S.S.
My dad taught me that when i was young. Hes been missing a chunk of his lip that they dug a chunk of battery lead out of since before i was born. Older man had filled his battery the night before and it froze. Blew up shortly after the jumper cables were hooked up.
I had a friend come over once to visit. When she got in her car to start it up to leave, the battery exploded with enough force to buckle the hood of the car up. 
I have been leery of batteries ever since. I am not sure that one would have even been survivable if you had been leaning over it.
Scary close call. Thanks for sharing the lesson.
Damn man, glad you are OK.
I had a battery explode once when I started the car. Motor had just started to turn over and WHUMP, the hood of the car had a bulge. It was fairly loud as I recall. I asked the guy at the battery place if I could get my core back on that. 
The battery dealer gave me full credit for the traded-in exploded core. 
This is like an episode of "Tool Time". "I just did that to show the viewers what could happen."
Glad you're OK.
When I was initially starting the 347 in the Fairmont, it was being somewhat cantankerous, requiring several recharges. I had the battery out of the car for charging while I did other stuff, so I plopped it down in there and dropped the cables on. The positive cable was a little tight going on, so I tapped it into place with the back of a screwdriver. When I hit the key, BAM! Blew acid all over my new engine, fender, engine compartment, etc. Aftet making sure the other two guys didn't get any on them, I commenced to rinsing the whole damn show with water, and rubbing what I could. It was a close call for all concerned, and an important learning experience. I'm also keeping an eye on that frame rail, to make sure it doesn't rust out from the inside.
Wow. I've generally been pretty careless around batteries, but I guess I'll be a little more careful going forward. I know they produce flammable gas, but I've never seen one blow up, or even heard of it happening among my many friends who work on their cars.
Two weeks ago I was pulling the battery out of my son's Miata, undoing the positive just nicked the body with a wrench but it didn't ground........OK dumbass stop being lazy walk back over to the tool box and get a the little short ratchet that won't hit anything.
I've had a battery go kablamo while driving a car and seen several them pop when other people were working on them. Things like grinding near them, wrenches speaking or hooking up jumper cables.
Tom
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