I have a relative who used to work for American, so I had the opportunity to fly on standby for a few years (D2 for anyone who plays the nonrev game). They have reciprocal agreements with a bunch of airlines around the world.
I was flying Iberia from Tel Aviv to Madrid once and just barrrrrrrely managed to squeeze on a flight. It was a 6am flight that I assumed would be empty but happened to get overbooked because of another canceled flight, so all those magical empty seats vanished before my eyes.
They managed to squeeze me on in one of the flight attendant jump seats - not the one in the cockpit, but the super uncomfortable vertical half-seats at the front of the plane. The only other standby passenger was the husband of one of the flight attendants.
Right after we took off, his wife came up to both of us and said “hey it’s 6am, you guys look tired - do you want to sleep in the crew rest?” I mean, why not?
So he took the bottom bunk and I took the top one - I think this was an A330 or 340 — it wasn’t nearly as fancy as the 787 here, but it was damn cozy. Pitch black, full lay flat, and I slept the whole 5 hours like it was 5 minutes.
Later a few friends told me this was absurdly against every regulation and that I basically had a once-in-a-lifetime experience. But I guess shrug Iberia Airlines, eh?