RC-135, way up North of the USSR near the Pole.
Nightime. Big 2 foot ball of St. Elmo's forms on the end of the radome and sits there for a while. Slowly, ever so slowly, it starts rolling towards the windscreen. We're thinking this is pretty cool.
It gets to the center bar dividing the two front cockpit windows and hangs up for a few minutes. Put you finger to the window and spidery lines trace out to your fingertip.
Then it starts to roll inside. Hmmmmmmmmmmm... what's this going to do. Rolls across the glareshield, down over the fire T-handles, drops to the center stand, rolls over the throttles and down the center stand to the floor.
At this point, the Pilots and Navs are all shrunk back away from it as far as possible. It's rolling aft, towards the cockpit door.
We ring up the EWO's (sitting on the other side) of the door on interphone and tell them to watch the bottom of the cockpit door and they'll see something pretty neat.
Ball hits the doors, eases through and disappears on our side. We wait. Nothing from the EWO's. A few minutes pass, EWO calls up and says "what are we looking for?"
The ball just disappeared as it sank into the door. Never showed on their side, didn't see any more St. Elmo's that flight.
Didn't bother the aircraft/instruments/anything as far as we could tell.