Another thread mentioned hazards while refilling O2 systems, and I think the subject is serious enough to warrant its own independent thread.

A few years ago, a visiting pilot came into my shop, saying his Oxygen transfill line was leaking. (He carries his own cylinder for refills.) I inspected the fittings, as they are the usual source of leaks, but he said, "No, it is leaking in the middle." Since it was a braided stainless steel line, I was skeptical. But on closer examination, I found two "burn spots" in the braid. They were about two inches apart, and I realized that they had been caused by the line shorting across his battery terminals. (The batteries were behind the cockpit on the shelf and the O2 cylinder was mounted farther back in the fuselage.)

I told the pilot to go buy a lottery ticket, as he is one of the luckiest people alive. If the line had been pressurized when the arc happened, I am pretty sure the combination of high-pressure oxygen, a lead-acid battery with a plastic case, aviation fuel present (it was a motorglider) and a lot of petroleum based epoxy in the aircraft structure would have brought a quick end to his charmed life.

Then we insulated his battery terminals with a few layers of heat shrink tubing and a shield that prevents unwanted contact across the terminals.

Something to watch for. Be careful. High pressure oxygen and the refilling process can be very dangerous. Make sure you KNOW what you are doing, and look out for subtle "gotchas" that are not readily apparent.