Have you ever added water to the car battery?

BIG ONE HERE: 80 posts regarding batteries and adding water and eye and facial safety hasn't been mentioned.
Opening cells, slamming push caps back down onto possibly overfilled cells or just jostling a battery over to your bench (whatever) can cause splashing. Lecture over

BIG REALIZATION: Snagglefoot played the main character in the hit Canadian TV series, "The Battery Inspector".
I knew I recognized his thoroughness.

REAL OCCURRANCE: The PO of my sister's 1984 Toyota Pickup (not a Hilux-that name started here in 1986) only checked 3 of the battery's 6 cells as a hold-down interfered with removing one of the 2 covers. Those neglected cells took over 12 ounces (>360ml) of water.
I told Sis, "When this battery shows any deficiency or fault....it's over".

ANOTHER REAL OCCURRANCE: The battery in my 1972 Toyota Corolla de Luxe broke free during a sweeping turn and the fan chewed off a corner, throwing a spray of acid into the air. It irritated my nose, eyes and throat quite a bit.
If you have an older vehicle, check your hold-down hardware.
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Years of pumping gas as a kid in the 70's and early 80's, adding water to batteries was a regular under the hood check at the full service pumps. Like others have mentioned we had the fill jug that stopped automatically when full. Then, and now, we used just tap water. It probably wouldn't hurt to get a jug of distilled to have around now since I'm on a well, but I don't have evidence either way that I killed a battery from it.

What Kira mentioned above though is beyond important. I experienced it first hand. Once I was using a water bottle to top off a battery. I'm looking over it, start to pour, and the water splashed right into my eye. At the moment, I was sure I lost my eye and was mad how stupid I was. I ran to the garden hose, and kept water pouring over it for a long time. It stung, but everything was fine. I suppose it was mostly just the poured water that splashed up, but lesson learned. Eye protection when servicing batteries.
Bought a 1 owner, 12 year old, 12k mile used vehicle in 2012. The battery was 3 yrs old. Within 2 months the battery was "dead." I found the cells uncovered and lots of sulfating. Apparently no one had ever checked it. I added around 16 oz of distilled water to that battery over a couple days and charged the heck out of it as well. It came back to life. Most of the visible sulphating went away. Got another 3 yrs out of it with weekly charges. Then put in a new battery in 2015. The cars sits a LOT. I trickle charge the battery every week and check water level twice a year. Minimal water added on this one. 8 yrs so far.....the longest life I've yet achieved on a battery.

If you can check the cell fluid level, do it.
Use a small funnel (tilt it) and a small filling bottle.....and you won't get splashed.