The first officer of Colgan Air Flight 3407, the one that crashed on approach to Buffalo with pilot fatigue being a factor according to the NTSB, deadheaded overnight in a FedEx jumpseat from SEA to EWR with a midnight to 4 AM layover in MEM.

This is super common. You'll regularly see OAL pilots and FAs on competitor metal flying to/from home. WN is really popular for this, because they offer very good prices for confirmed space for crew members and otherwise basically free for standby.

I believe EI has some US based crews, though I wouldn't be surprised to see some deadheading on that.

And it still wasn't her who failed multiple checkrides and ignored the stick shaker. She almost saved the whole thing.

On a recent AA flight, a uniformed Spirit Airlines FA was onboard--in a MCE seat, too. As others say, this is normal; however, I immediately thought, "wow, you must be in heaven compared to your usual legroom." :-) It was an on-time (early) flight and everything. Full beverage service (fewer than 500 miles so nothing fancy, but I am thinking she loved it).

Normal to see an Allegiant pilot or two on my Delta flights out of VPS, where Allegiant has a pilot base, and Allegiant’s non-daily route networks mean it’s often a lot easier to reposition pilots and FAs if they put them on other airlines.

I'm an equal-opportunity jumpseater. Follow the chairs.

had to go ATL-BOS a few weeks ago ... we cancelled ATL-TLV the night before. There wasn't an ATL-TLV the next day. So VIPER rebooked everyone ATL-BOS-TLV. The first several ATL-BOS flights went from ~15 ish open, to oversold. plus the usual litany of Delta nonrevs departing Atlanta on the first flight of the day. Went over to Spirit; showed "full." waited til the end: Big Front Seat! I was the only NRSA / JS.

I have found over 20 years of doing this, that with 95K employees at your company, its usually the other NRSA that will sink you. It pays to get out of the stream. G4/F9/NK and even WN on many routes just don't have the volume of standbys.

The problem with this is that you'll never get a PDB, and without such, I figure you might as well quit :D

When I was flying in the military I would often see uniformed Army soldiers and uniformed Navy sailors flying together- on an AIR FORCE plane - believe it or not.

Space-A is a whole other world... I am kind of surprised there isn't a Space-A forum here on FT for it to be honest. Full of its own lingo and cheat codes, flight hacks, etc.