Had one near miss long time ago on a weary DC08-21 at JFK. Radio Mech went out to check something and turned on the Master radio switch. Blue flash in cockpit, his eyebrows were singed and, being a radio techie, had no trace of oil on him..
I checked log book, found rather frequent refiling of crew oxy.
I ordered a complete bubble juice check of every crew oxy line. Having torn out three quarters of cockpit line they finally found it. It appears leaking oxy accumulated behind liner and radio switch caused it to eat up all the accumulated dust. No progression though, no damage except trying to put all the petrified liners back on.
Remember lots of aircraft types had external oxy fillers. Fitting was a sweated on small fitting. There was a section of tubing with a mesh inside to dissipate heat. Was rarely used in my experience. Never could find fitting.
Eastern Air had an oxy fire when charging a removed bottle. The equipped a clean room at each station complete with water tubs to keep bottles cool as they were filed.
Did get a bottle filled by an approved vendor that almost knocked out F/E when pulled an oxy check with his mask. Had it empty sent to me and smell from it was noxious. Apparently the shut iff valve on the bottle had a base seal and it combusted during a too rapid filling. Would have been fatal if crew had needed it in flight. Added a "sniff check" to our procedure whenever fitting a refilled bottle. Crack open just a slight bit and wave the gas to you. No problem to detect a bad one.